\documentstyle[11pt,avm]{article}


\setlength{\evensidemargin}{0in}
\setlength{\oddsidemargin}{0in}
\setlength{\textwidth}{6.5in}
\setlength{\topmargin}{-0.5in}
\setlength{\textheight}{9in}
\newcommand{\hidden}[1]{}

\newtheorem{definition}{Definition}

\newcommand{\transarrow}{\ \Longleftrightarrow}

\newcommand{\typef}{\it} 
\avmfont{\sc}
\avmsortfont{\scriptsize\it}
\avmvalfont{\it}

\newcommand{\singlespace}{}


\newcommand{\etal}{{\it et al}}


\newcounter{examplectr}
\newcounter{subexamplectr}
% Example Macro
\newenvironment{ex}%
   {\vspace{.1in}\addtocounter{examplectr}{1}
     \setcounter{subexamplectr}{0}
     \begin{list}
       {(\arabic{examplectr})}%
       {\setlength{\topsep}{0in}
        \setlength{\leftmargin}{.25in}
               \setlength{\labelsep}{0.075in}}
       \item \begin{minipage}[t]{6in}
   }%
   {\end{minipage}
    \end{list}\vspace{.1in}}

% Sub-example Macro
\newenvironment{subex}%
   { \addtocounter{subexamplectr}{1}
     \begin{list}
       {\alph{subexamplectr}}%
       {\setlength{\topsep}{-\parskip}
	\setlength{\leftmargin}{0.175in}
	\setlength{\labelsep}{0.075in}}
       \item
   }%
   {\end{list}}
%

\newcommand{\exnum}[2]{\addtocounter{examplectr}{#1}(\arabic{examplectr}{#2})\addtocounter{examplectr}{-#1}}

\begin{document}
\begin{center}
{\large\bf Semantic transfer for Verbmobil}\\
Draft\\
Ann Copestake\\
University of Stuttgart and CSLI\\
\today
\end{center}

\section*{Abstract}
This paper is a detailed discussion of semantic transfer in the context
of the Verbmobil Machine Translation project.  The use of semantic
transfer as a translation mechanism is introduced and justified by
comparison with alternative approaches.  Some criteria for evaluation
of transfer frameworks are discussed and a comparison is made of three
different approaches to the representation of
translation rules or
equivalences. This is followed by a discussion of control
of application of transfer rules and interaction with a domain
description and inference component.

\section{Introduction}

Verbmobil is a spoken-dialogue machine
translation system, which takes German 
language input and produces English output.  In the
near future it will also operate with Japanese input and English output.
The system currently assumes a small vocabulary 
and is restricted to the limited domain of arranging meetings
but intended to be extensible (in the medium to long term) 
to less restricted topics.  Because of this ambitious long term goal, 
components of Verbmobil must be well motivated in design, to attempt to
avoid, or at least delay, the problems in scaling up which have so consistently
hit previous speech and natural language systems.

In the current paper, we describe a range of approaches to translation
and discuss their suitability for Verbmobil, taking account of both
the medium and the long term goals.  We will attempt for the most part
to abstract away from existing implementations, 
but to keep the discussion grounded
by considering examples taken from the task-oriented
dialogues collected for Verbmobil.
However, as far as possible, we will attempt to generalise from these examples
to classes of problems that might arise, both within the chosen domain
and with a less restricted task.  
The intent of this paper is to follow-on from
Kay \etal\/\ (1994) and to make a more detailed and concrete evaluation of
approaches to MT 
in the light of experience already gained on Verbmobil (and on other projects).
We will assume the use of HPSG for
the syntactic representation (Pollard and Sag, 1994), 
DRT for the semantics (Kamp and Reyle, 1993) and an underlying
unification/constraint-based implementation framework.
We will only consider German to English translation specifically here,
due to lack of sufficient Japanese data for the Verbmobil domain.

This paper is organised as follows: in \S\ref{review} we briefly review 
existing approaches to translation.
Following Kay \etal\/\ (1994) we will argue that
the most appropriate style of approach is one in which analysis produces a 
(underspecified) semantic representation, which is not expected to
be identical to the semantic representation for the target language.
We will refer to this as semantic transfer, but we construe this rather broadly.
As we will explain, we include systems which make some reference to
syntactic structure as well as semantics and also approaches which make use 
of a (partial) interlingua but do not insist that a 
single representation exists
for source and target utterances.  We also include systems based 
on semantically driven lexicalist
transfer.
We will diverge somewhat from Kay \etal 's suggestions in that we
assume a rather 
greater role for transfer and less reliance on interlingual representation.
In \S\ref{criteria} we introduce some 
criteria for evaluation which will be used in a more detailed comparison of 
three particular approaches to stating transfer rules.
In \S\ref{mds}, \S\ref{tlinks} and \S\ref{mrs}
we describe the three approaches in 
detail, by illustrating how they work on some sentences from the Verbmobil
dialogues.  We use other examples to examine different facets of
translation and expose some limitations in the approaches.
In \S\ref{inference}, we discuss the possibilities for interface with a
domain model and inference engine with respect to these approaches, and
outline ways in which the application of transfer rules
may be controlled.




\section{A classification of approaches to MT}
\label{review}


The following description of MT approaches is intended more as
an indication of the nomenclature we will use than as a full discussion, for
which see e.g.~Hutchins and Somers (1992). Kay \etal\/\ (1994) 
discuss these approaches in the Verbmobil context.
The references to various systems given below are not intended to
represent an absolute categorisation, since the boundaries between
these approaches are not sharp.  In particular, some systems have ways
round the particular disadvantages which we identify, but this
generally involves adopting some techniques from other approaches. 
\begin{description}
\item [Direct transfer] (e.g.~SYSTRAN, though this
has further refinements; 
nearly all portable and PC based systems)
Very limited processing of the source language (SL) string is followed
by transfer to a target language (TL) string.  
Some morphological analysis is usual
but no parsing or semantic analysis takes place.  The limitations are obvious:
vast numbers of transfer rules are required for anything other than very 
limited and repetitive text, the only disambiguation is based on collocations,
the output is likely to be ungrammatical. 
SYSTRAN itself is not as limited as this description would suggest,
since the current version incorporates some deeper analysis. 
\item [Syntactic transfer] (e.g.~the METAL
system, Slocum \etal, 1987)  
Transfer is carried out on syntactic trees.  It therefore
requires that a syntactic analysis of the input be possible, which
requires a significant amount of processing, and a large grammar and
lexicon (for a broad coverage system).  
The transfer component will have
fewer rules than in the direct transfer method, but the rules are more
difficult to write, since they must refer to abstract syntactic
categories rather than surface strings.  
The degree of similarity between source and target representations
depends on the syntactic analysis.  However, 
a syntactic transfer approach makes little sense when using
HPSG, since the phrase structure trees corresponding to the source and
target languages
will often be very different, and the semantics is constructed in
parallel with the syntax. 
\item [Unification-based syntactic/semantic transfer] (e.g.~Zajac (1989), 
Kaplan \etal\/\ (1989), ELU --- Estival \etal\/\ (1990), 
MiMo --- Arnold and Sadler (1990)) 
Unification-based systems can
treat transfer rules as being similar to grammar rules, which
map between feature structures. 
Such systems have many of the good points of
unification-based grammars: they are (to a large extent) declarative
and can therefore accommodate different parsers and generators,
and they can be bidirectional.   
The systems mentioned above rely mostly on stating semantic
relationships:
it is not entirely clear
whether the capability to express syntactic mapping is ever really
necessary, because where reference to syntax is made
it could be argued that this is really necessary only
because of deficiencies in the monolingual 
semantic analysis.
The complexity of the transfer rules depends on
the
syntax of the
semantic representation.  
We will discuss this in more detail below.
\item [`Pure' semantic transfer] (e.g.~BCI --- Alshawi \etal, 1991) 
Transfer is carried out
between semantic representations.  
The distinction between this and the previous class is not clear-cut,
however, as some syntactic information may also be accessed.  The
BCI system, for example, uses information about whether a sentence is
active or passive in transfer.
The use of the term
`transfer' implies that the mapping is non-trivial --- i.e.~that the
semantic representations of the source and target language are not
identical and may use different predicate names.  However, there is no
clear distinction between semantic transfer and an interlingua system that
does not insist that source and target semantic representations
are identical, since most interlingual systems have some language specific
predicates and most semantic transfer approaches make use of some
interlingual concepts (to represent tense, for example).
We will discuss pure semantic transfer in more detail below.
\item [Interlingua] (e.g.~Dorr (1994), Emele \etal\/\ (1992), Landsbergen (1987))
In the prototypical (`naive')
interlingua approach, an identical representation is
always used for source and target language utterances.  One advantage
which is generally claimed for this is that while
transfer between $n$ languages requires that $(n-1)^2$ 
sets of transfer rules be built, an interlingua approach only requires
$n$ sets (one for each language) mapping to the common representation.
However, this argument implies that an interlingua 
is not an asset in translation between two languages 
(one transfer rule-set vs. two
language-to-interlingua rule-sets)
or for the
Verbmobil scenario (two unidirectional transfer rule-sets vs. three
language-to-interlingua sets).
Below we discuss some reasons to doubt that this pure
form of interlingual approach can be achieved in any case.
\item [Lexicalist transfer] (Shake-and-Bake --- Whitelock (e.g.~1992)
and variants: e.g.~Copestake \etal\/\ (in press), Trujillo (1995)) 
This approach relies on the use of a lexicalist unification based
grammar formalism.  Translation equivalences are
stated by relating the semantics of
source and target language lexical signs.  It has been claimed that
this combines the advantage of simplicity of description of transfer
rules found in the direct approach, with some of the advantages of
semantic or syntactic/semantic transfer.  This is also discussed in more
detail below.  
\item [Knowledge based machine translation] (e.g.~Nirenburg and Raskin, 1988)
This approach emphasises reliance on domain knowledge in carrying out
translation.  It is based on extensive (and time-consuming) domain modelling.
It has been used
for tasks such as translation of technical manuals written in a subset
of a
natural language which is constrained with respect to vocabulary and grammar.
The approach is often claimed to be an interlingual one, but the 
language independent representation is domain-specific, 
and some transfer rules may be used for convenience.  The problem with this
approach for Verbmobil is its domain and task specificity --- while it
is reasonable to suggest that technical manuals
refer to a strictly circumscribed
domain, it is much more difficult to defend this position
with respect to the Verbmobil dialogues and the meeting arrangement task.
It is possible to regard meeting arrangement in an abstract
way as steps in a process to narrow down times and locations to achieve
compatibility.
However,
it is clear that a lot is going on in the Verbmobil dialogues which is
non-essential and unpredictable from this viewpoint, 
but is an inherent part of human communication.  For example, excuses
for not being able to meet at a particular time cannot be treated as part
of a domain model, because the possible excuses are unbounded.
Therefore, we regard domain knowledge as an important component in a
translation system, but do not believe that basing the system
completely on the domain is a good long term strategy.  
\item [Statistical approaches] (e.g.~Brown \etal, 1990)
These are only relevant to a situation where a very large bilingual
corpus exists, and only work well in situations where very local
context gives enough information to translate a word.  For these
reasons, such approaches appear inapplicable 
as the main approach to translation in Verbmobil 
and thus
we do not
consider them further here (see Kay \etal\/\ for further
details).\footnote{Statistical approaches may have a role to play in
control of translation, however, see \S\ref{control}.}
\end{description}

\subsection{Triangles, polygons and naive interlingual approaches}

The classic way of describing the tradeoff between some of these approaches to
translation is the Vauquois triangle reproduced in Figure~\ref{vauq} 
(adapted from Hutchins and Somers (1992)).  
The vertical dimension is intended to represent the degree of difficulty
in analysing the source to the chosen level and 
in generating the target string
from the corresponding target language representation.
The horizontal dimension is intended to represent the degree of difficulty 
in carrying out transfer (i.e.~converting the
analysed source representation to the target representation).  
On this picture, the
direct approach takes the least effort for analysis and generation, but
actual transfer is most difficult.  In contrast, the apex of the 
triangle represents a naive interlingual system in which 
a single representation is constructed for 
the SL from which the TL string can be generated. This
makes transfer unnecessary but increases the difficulty of analysis and
generation.  Syntactic transfer and semantic transfer are seen as 
being on intermediate points in the scale.
\begin{figure}
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(110,80)(-5,-3)
\thinlines   
              \put(0,0){\line(1,0){100}}
              \put(5,2){\vector(1,0){90}}
              \put(50,5){\makebox(0,0){direct}}
              \put(18.3,20){\vector(1,0){63.4}}
              \put(50,23){\makebox(0,0){syntactic transfer}}
              \put(30.3,38){\vector(1,0){39.4}}
              \put(50,41){\makebox(0,0){semantic transfer}}
              \put(0,0){\line(2,3){50}}
              \put(100,0){\line(-2,3){50}}
              \put(-5,0){\vector(2,3){50}}
              \put(5,35){\makebox(0,0){analysis}}
              \put(55,75){\vector(2,-3){50}}
              \put(95,35){\makebox(0,0){generation}}

            \put(0,-3){\makebox(0,0){Source Language}}
            \put(100,-3){\makebox(0,0){Target Language}}
            \put(50,78){\makebox(0,0){Interlingua}}
\end{picture}
\end{center}
\caption{Vauquois triangle}
\label{vauq}
\end{figure}


Given the current state of the art in natural language
processing, we believe that a more accurate picture is that
shown in Figure~\ref{newvauq}.  
\begin{figure}
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(110,80)(-5,-3)
\thinlines   
              \put(0,0){\line(1,0){100}}
              \put(5,2){\vector(1,0){90}}
              \put(50,5){\makebox(0,0){direct}}

              \put(15,20){\vector(1,0){70}}
              \put(50,23){\makebox(0,0){syntactic transfer}}

              \put(39,35){\vector(1,0){22}}
              \put(50,40){\makebox(0,0){\shortstack{semantic\\ transfer}}}

              \put(100,28){\makebox(0,0){underspecified semantics}}
              \put(100,48){\makebox(0,0){resolved logical form}}

              \put(0,0){\line(1,2){10}}
              \put(100,0){\line(-1,2){10}}
              \put(10,20){\line(3,1){24}}
              \put(90,20){\line(-3,1){24}}
              \put(34,28){\line(1,2){10}}
              \put(66,28){\line(-1,2){10}}

              \put(44,48){\line(1,5){1}}
              \put(56,48){\line(-1,5){1}}

              \put(46,58){\line(1,5){1}}
              \put(54,58){\line(-1,5){1}}

              \put(48,68){\line(0,1){4}}
              \put(52,68){\line(0,1){4}}


            \put(0,-3){\makebox(0,0){Source Language}}
            \put(100,-3){\makebox(0,0){Target Language}}
            \put(50,78){\makebox(0,0){Language Neutral Utterance Representation}}


\end{picture}
\end{center}
\caption{Modified Vauquois `triangle'}
\label{newvauq}
\end{figure}
This diagram is not intended to be
taken too seriously, but expresses several divergences from Vauquois'.
The most important of these is
skepticism about the possibility of a single language-neutral 
sentence representation.
If there is a possible language neutral representation, it is at the
level of the utterance, rather than the sentence: i.e.~context must be
taken into account.  But current NLP systems have only made very limited
progress towards this sort of analysis.  MT is feasible because the
speaker and the hearer to a large extent share the same world knowledge
and context model, so the translation system can avoid doing some of
the hard work, if the translated sentence 
makes the ``correct'' set of inferences available to the hearer.
Thus a literal translation of {\it the window is open} into German
would probably be interpreted by the hearer as a request to close it
if the English utterance would be interpreted in that way.  
But an MT system
would be unable to accurately decide whether the sentence should
be interpreted as a request, because so much world knowledge could
influence this: room temperature, noise level, odours etc etc.

Kay \etal\/\ (1994) discuss the difficulties with the
naive interlingual approach
in some detail, so here we simply summarise their
argument, which is basically
that some sentences cannot be idiomatically
translated in a way that completely preserves
meaning.  One example given by Kay \etal\/\ (1994:26) is:
\begin{ex}
English: He asked where he should stand.\\
French: Il a demand\'{e} o\`{u} il devait se mettre.\\
Lit: He asked where he should put himself.
\end{ex}
In this case, the French example is less specific than the English
source, but a more literal translation would be awkward.

An example from the Verbmobil dialogues, which illustrates the same
point, is given in \exnum{+1}{}: 
\begin{ex}
German: Da habe ich immer schon Feierabend\\
Lit: Then I have always already time-after-work\\
English: I have (usually) finished work by then
\end{ex}
There are two sources of difficulty in 
finding a good translation: the noun
{\it Feierabend} does not have a straightforward equivalent in English
({\it knocking off time} is the closest, but is point-like while 
{\it Feierabend} refers to an interval, and {\it knocking off time}
is mostly used for factory work)
and it is difficult to find a way to convey the same force of
the habitual (which is less strong than the literal translation by {\it
always} would suggest).  In many contexts, the English sentence given would be
equivalent to the German one, but it is more general, since {\it finish
work} could refer to work other than an actual job, or to a time period
other than the whole day (e.g.~that sentence could be used in a
different context to refer to the time a lunchbreak started).
It also illustrates a general problem which can occur if a noun is
not translated as a noun: subsequent pronominal reference is possible
to {\it Feierabend} in the German discourse, but not in the English:
\begin{ex}
\begin{subex}
Speaker A: Da habe ich immer schon Feierabend\\
Speaker B: Oh, in meiner Firma ist er sp\"{a}ter.
\end{subex}
\begin{subex}
Speaker A: I have (usually) finished work by then.\\
Speaker B: ? Oh, it's later at my company.
\end{subex}
\end{ex}
It is therefore clear that the sentences in \exnum{-1}{} cannot have
the same DRS.

There are other cases of lack of translation equivalence which
involve cultural differences.   For example,
in 
the Langenscheidts Taschenw\"{o}rterbuch,
{\it Assessor} is given a definition in English rather than a translation:
{\it civil servant who has completed his/her second state exam}.
Such examples are frequent in bilingual dictionaries 
and cause considerable problems when trying to construct a transfer
lexicon automatically (Copestake \etal, in press).  However, these are
not strictly speaking translation problems, since German 
language speakers from
another country would have the same difficulty in understanding {\it
Assessor} as English speakers would. One of the few cases of a clear cultural
difference that shows up in the Verbmobil dialogues is the occasional
reference to {\it Kalenderwoche} (weeks of the year):
\begin{ex} 
German: Im M\"{a}rz h\"{o}chstens die zehnte Kalenderwoche aber nur
ungern.\\
Lit: In March at best the tenth calendar week but only reluctantly.
\end{ex}
The concept could be expressed precisely in English, of course, but 
only in a very long-winded way, because there is a convention as to
when the first {\it Kalenderwoche} begins which has to be explicitly
expressed.  It is not a
usual way of referring to dates in either the UK or the USA and even in
Germany is only usual among businessmen.
These cases are
clearly different from the 
{\it Feierabend} example, since that concept is universal
in Western industrialised countries but is simply not lexicalised in
the same way in English.  

It may always be possible to
express the same truth conditional 
meaning in another language by means of some 
complex paraphrase, but this is irrelevant to the problem of translation,
since it is unacceptable to produce verbose TL strings 
given a simple SL
input.  This is particularly true of spoken dialogue systems.
A translation which was meaning preserving but verbose would fail to
serve the communicative function of the utterance.

Thus, as we said at the beginning of the section,
if there is a language neutral representation, it is at a different
level: one in which the communicative intentions of the speaker are
modelled.
\footnote{This 
does not apply to systems where the application is much more
circumscribed and conventionalised, such as translation of weather reports 
and of
the restricted language technical manuals mentioned earlier. Here a
language-independent representation may be 
possible, though highly task-specific.}
However, as the dashed lines 
in Figure~\ref{newvauq} are intended to convey, we have 
relatively little knowledge of
how to analyse a discourse to achieve a model of the speaker's communicative 
intentions,
compared to our knowledge of syntactic and semantic analysis. 
As the gradient of the 
lines is intended to suggest, it is certainly difficult and
computationally expensive, even if it is achievable in principle
with a restricted domain. 
Furthermore, the current assumption about the use of Verbmobil is that
the machine will only be required to translate parts of a dialogue and 
that, because
of the limitations of speech recognition technology, it will
not be able to monitor portions of the dialogue it is not required to
translate.  Even without this limitation, it will, of course, miss any
non-verbal communication (gestures etc) which the participants in the
dialogue make.
It is therefore
unrealistic to assume that Verbmobil could maintain a
sufficiently accurate discourse 
model for a language-neutral utterance
representation to be possible.

This is not to deny that context is important in translation.
There are cases where it is impossible to translate without having
some model of the discourse state and the speaker's intentions.  
For example, within the Verbmobil dialogues,
there are two quite different possibilities
for translation of \exnum{+1}{a} into English,
shown in \exnum{+1}{b} and \exnum{+1}{c}:
\begin{ex}
\begin{subex}
Geht es bei Ihnen?
\end{subex}
\begin{subex}
Does that suit you?
\end{subex}
\begin{subex}
How about at your place?
\end{subex}
\end{ex}
We will therefore argue for a model in which domain and discourse
information can be accessed by transfer.  Initially this access would be
quite limited, but as techniques for discourse analysis and speech
recognition improve, we would expect transfer to rely more heavily on
context.

There are also straightforward engineering reasons for rejecting
approaches which require identity of source and
target language representation, because they require that the source and target
language grammars are developed together, in order to ensure the
meaning representations are identical (or at least compatible).
It is quite difficult enough to build a robust grammar without this
constraint.  
For Verbmobil it is practically impossible to ensure that 
English and German grammars produce identical representations,
since the grammars
are being developed by different sites.  Building extra modules, to convert the
representations produced by the grammar into a unique interlingual
representation, is also unattractive, since it would essentially
require three transfer components instead of two (German, English and
Japanese to interlingua, as opposed to German-to-English and 
Japanese-to-English).

We therefore reject a model of translation in which it
is necessary to produce a single representation, in favour of one in which the
source and target representations may differ.
The assumption is that
translation can be accomplished
in many cases without attempting a full calculation of
communicative intention, and thus that transfer at the
level of semantic representation of the sentence
is the most sensible option, provided that the transfer component
can interact with other modules in the cases where this fails.  
Thus, for \exnum{0}{a}, the transfer component might query some
other module
which would not have to maintain a full discourse model, but would have
to discriminate between a state of the dialogue where
agreement on a prior proposal was
sought, where \exnum{0}{b} 
would be appropriate, and another state where a location for the 
meeting was being suggested, where \exnum{0}{c} would be required.

\subsection{Underspecification}
\label{underspec}

The second main point of Figure~\ref{newvauq} is that it
is possible to have underspecified semantic representations.
This has been discussed by Alshawi \etal\/\ (1991) with respect to
the decision to use QLF (quasi-logical form) as the transfer level in
the SRI BCI system, rather than the fully instantiated logical form.  
The general issue is that disambiguation can involve an
arbitrary amount of reasoning on real-world knowledge, which is highly
problematic in an implementation.  If the ambiguity is preserved,
however, the pragmatically plausible reading will be the same in the
target language as in the source, and the hearer will normally get the
right reading without conscious effort.
Several kinds of underspecification are potentially relevant:
\paragraph{Attachment ambiguities}
PP-attachment ambiguities 
are notorious problems in analysing English,
since the number of possible parses for a sequence of PPs increases
according to the Catalan series, and disambiguation can involve
arbitrarily complex reasoning.  Sequences of four or more
PPs are not uncommon.  Similar remarks apply to German.  
In German to English translation, if PPs can be translated
straightforwardly as PPs, the preferred reading will usually remain the same if
the order is maintained. 
\begin{ex}
\begin{subex}
Jeder Nachmittag ist frei in meinem
Terminkalender ab vierzehn Uhr.
\end{subex}
\begin{subex}
Every afternoon is free in my
appointment book after two o'clock.
\end{subex}
\end{ex}
Similar remarks apply to other attachment ambiguities, such as those
involving coordination:
\begin{ex}
\begin{subex}
Meinen Sie Donnerstag den 8.~oder Donnerstag den 15.~Juli?
\end{subex}
\begin{subex}
Do you mean Thursday the eighth or Thursday the fifteenth of July?
\end{subex}
\end{ex}
It is pragmatically most plausible that {\it Juli} has wide scope
here, but {\it den 8} could refer to April 8th.  However, it is
unnecessary to attempt this disambiguation, because the hearer is
almost certain to get the right reading, because he or she has the
appropriate context and world knowledge.
It should be noted that this requires an
interface with syntax which allows attachment to remain
underspecified.  
\paragraph{Quantifier scope}
In contrast, quantifier scope ambiguities do not correspond to
different syntactic structures, although syntax does play a role in
determining which readings are possible.  In \exnum{+1}{} is a (made-up)
example, where dual readings are plausible (at least for English).
\begin{ex}
\begin{subex}
Jeder Student in diesem Studiengang muss vier Programmiersprachen
lernen.
\end{subex}
\begin{subex}
Every student on the course must learn four programming languages.
\end{subex}
\end{ex}
Conventional grammars give some scopes which are not at all
plausible to human readers but excluding implausible
readings is not straightforward. 
Thus excluding the unwanted reading from \exnum{+1}{}
(which is taken from the CMU collection of English dialogues)
is not totally trivial:
\begin{ex}
I have some time almost every day.
\end{ex}

We will discuss one technique for
the representation of underspecified scope in \S\ref{mrs},
but here we should point out that 
quantifier scope ambiguities may not be too serious for
Verbmobil, since 
German word order usually disambiguates scope
and potential ambiguities are relatively rare in the Verbmobil domain:
\exnum{0}{} is one of the few examples found with an explicit
universal, for instance. 


\paragraph{Pronouns}
Pronoun resolution is another area in which current NLP systems cannot
achieve perfect results, although the algorithms are generally better
than those available for PP-attachment.  For German-to-English
translation, pronouns can be left partially resolved: that is, it is
necessary to know whether a third person pronoun refers to a human or
not (to avoid {\it er} being translated as {\it him} when it actually
refers to a {\it Termin}, for example) but it is unnecessary to know
which human or object it refers to.  Thus, it is unnecessary to resolve
{\it es} in \exnum{+1}{a} in order to translate it to \exnum{+1}{b}:
\begin{ex}
\begin{subex}
Zur Not k\"{o}nnen wir es auf den ersten verlegen.
\end{subex}
\begin{subex}
If necessary, we could move it to the first.
\end{subex}
\end{ex}
In fact, pronouns are mostly
first and second person in the collected dialogues.
\paragraph{Compounds}
Attachment ambiguities are a problem with compounds with more than two
constituents.  But, quite apart from this,
it is desirable to be able to
underspecify the relationship between the parts of the compound,
because determining the correct relationship is potentially arbitrarily
complex.  How serious this is for Verbmobil 
in its current state depends on whether the
vocabulary limitation is taken to apply to compounds (e.g.~if {\it
Termin} and {\it Zahnarzt} are in the vocabulary, is there a
requirement to recognise 
{\it Zahnarzt-Termin} even if it is not listed explicitly?).
If productive compounds can occur
they should be translated as compounds in English where possible.  
For example:\\
{\it
\begin{tabular}{ll}
Mittags-Termin & midday meeting\\
Berlin-Videos & Berlin videos\\
Fasanengarten & pheasant garden
\end{tabular}}\\
However, this strategy will not work in all cases and it may
be necessary to resolve the relationship:\\
{\it
\begin{tabular}{ll}
Terminvorschlag & proposal for a meeting
\end{tabular}}

\paragraph{Lexical ambiguity}  Ambiguity between
homonyms, such as {\it
bank}\/:`side of a river' as opposed to {\it bank}\/:`financial
institution'
will nearly always have to be resolved in translation, since pairs of
languages will only share homonyms occasionally.
But regular polysemy is often language independent (in Apresjan's (1973)
collection of examples of regular polysemy, about two-thirds of the
patterns had counterparts in both Russian and English).  For example,
{\it Zeitung} and  {\it newspaper} show the same 
physical object/organisation polysemy.  More relevant to Verbmobil are
the unestablished cases of polysemy: for example \exnum{+1}{a} and
\exnum{+1}{b} both have the same metonuymic interpretation which is
given in \exnum{+1}{c}:
\begin{ex}
\begin{subex}
Nach Stuttgart treffen wir uns in Saarbr\"{u}cken.
\end{subex}
\begin{subex}
After Stuttgart we will meet in Saarbr\"{u}cken.
\end{subex}
\begin{subex}
After the meeting in Stuttgart we will meet in Saarbr\"{u}cken.
\end{subex}
\end{ex}
For further discussion of
regular polysemy see Copestake and Briscoe (1995) and references therein.

\subsection{Semantic transfer}
\label{geben}

Figure~\ref{newvauq} expresses a considerable bias in favour of 
(underspecified) semantic transfer
rather than syntactic transfer, in that constructing an underspecified
semantic representation involves relatively little effort 
for its return in narrowing the gap
between representations.
Again this has been discussed elsewhere,
including Kay \etal\/\ (1994), Alshawi \etal\/\ (1991).  
Here we simply make two points:
the first is that in HPSG 
syntactic and semantic processing
are very directly interrelated, thus in 
computational terms the extra overhead involved in 
building a semantic representation
may not be great.
How much effort is involved will depend on the 
form of the semantic representation chosen and how directly it relates to the
syntax.  
Secondly, the closeness in representation 
between source and target languages that is achieved will
also depend on the semantic representation.  
If very different representations are 
chosen, it is possible that the sides of the `triangle' could actually
diverge rather than converge.  
Again it is often the form of the
representation which is crucial here, and not the model theory.


Our conclusion in favour of
semantic transfer is broadly in line with Kay \etal's suggestions
for Verbmobil, though we have used some slightly different terminology.
We also accept their proposal for translation by negotiation, 
but in this paper we will concentrate on the declarative aspects of some
existing formalisms.  We have no firm proposals to make about how 
to achieve the (highly 
desirable) aim of very fluid interaction between components envisaged by Kay,
though we will note some potential limitations of existing formalisms in this 
respect.
One difference, however, is that Kay's model does not include 
any notion of transfer rules or equivalences and relies on the use of 
an interlingua.  The generator is required to approximate the source language
representation as closely as possible, but it is not clear how this
is to be achieved.
We think that it is necessary to give 
more guidance to the generator, and the best way to do this 
is to take advantage of knowledge about divergences
between languages and to write language pair
specific rules to transfer between source and target languages.

We also feel uneasy about the use of an interlingua, because
it implies strict
identity between concepts in different languages which is not
necessarily plausible, except for terms which have externally imposed
definitions.
For example, it is reasonable to assume that literal usages of
German {\it Elefant} 
and English {\it elephant} might have identical denotations, 
because they both refer to the same two species of mammal, but this 
is only clear for (some) natural
kind terms.  Sometimes differences in meaning are very
subtle: {\it geben} is normally translated as
{\it give} when it refers to a transfer of
possession, but it is sometimes used in contexts where {\it lend} would be
a much more appropriate translation in English\footnote{Thus
{\it Du gebst mir das Buch} 
could be said when the speaker had borrowed the book
two days ago but {\it You gave me that book} is judged to be
inappropriate by most English speakers
in this situation.  This use is
not equivalent to examples where {\it give} is used 
instead of {\it pass}, (e.g.~{\it Give me your plates so I can put them in the
dishwasher}) which is also possible in German.
It is more similar to the use in {\it I gave him my watch while I went
swimming}, but this is more restricted in English than the German use
of {\it geben} is, since it requires a very explicit context to limit
the duration of the transfer.
Thanks to Susanne Riehemann
for a detailed discussion of this.}.  
Such mismatches are not going to
appear in bilingual dictionaries, and it is probable that they will not become
apparent 
until a particular context is encountered that displays the difference.

If the {\it geben}/{\it give}
mismatch was discovered after we had associated an interlingual
term GEBEN with the entries for (particular senses of) {\it give} 
and {\it geben}
in the monolingual lexicons, we would be forced to change those entries. 
One way of doing this would
be to invent another sense for {\it geben} which would correspond to
the same interlingual predicate as {\it lend}.  
On monolingual grounds, however,
this does not seem to be justified, and it is unrevealing --- we would need to
be able to disambiguate between the senses in order to translate, so we
would need an explicit statement of the differences between them.
As an alternative, we could make
the semantics of {\it give} more complex, as shown below (this analysis is
given for the sake of concreteness, but is not
fully adequate):
\begin{ex}
geben: $\mbox{GEBEN}(e,x,y,z)$\\
give: $\mbox{GEBEN}(e,x,y,z) \wedge 
\mbox{\it NOT}(\mbox{\it BELIEVE}(\exists e' 
[\mbox{\it GEBEN}(e',y,x,z) \wedge \mbox{\it AFTER}(e,e')]))$
\end{ex}
This is intended to express the idea that {\it geben} might be neutral
with respect to future transfers but that {\it give} implies permanence.
In principle, we might want to explicitly state information like this about 
the meaning of {\it give}, but 
there is no 
monolingual reason to put it in the lexicon rather than treat it via 
inference rules.  There is no obvious bound on the complexity that might be
involved, and since this strategy involves using other interlingual predicates
in the lexical entries, changes could have ramifications
throughout the lexicon (e.g.~an English native speaker could have used GEBEN in
other entries assuming it was equivalent to {\it give}).

Extrapolating from the example given in Kay \etal\/\ (1994), we assume that, in the
translation by negotiation approach, the generator would
be passed a logical form containing 
$\mbox{GEBEN}(e,x,y,z)$ and then discover
that the `nearest' lexical entry was {\it give}
but that this carried some extra implications.\footnote{There are
difficulties in formalising the concept of `nearness', but we
will ignore this issue here.}  
The generator would
then check with the negotiator to see if these extra conditions held.
But a transfer-based approach could provide greater modularity.
There would be
language specific predicates for {\it geben} and {\it give}
which would be related by a simple transfer rule:\\
geben: $\mbox{\it geben}'(e,x,y,z)$\\
give: $\mbox{\it give}'(e,x,y,z)$\\
$\mbox{\it geben}'(e,x,y,z) > \mbox{\it give}'(e,x,y,z)$\\
The exceptional cases could be represented as follows:\\
$\mbox{\it geben}'(e,x,y,z) > \mbox{\it lend}'(e,x,y,z)$ \\
$\mbox{\it BELIEVE}(\exists e'[\mbox{\it geben}'(e',y,x,z)\wedge
\mbox{\it AFTER}(e,e')])$\\
Here the second line is a condition on the applicability of the rule.
Such transfer rules have a nonmonotonic interpretation, since 
they may fail.  The more specific rule
should apply in preference to the
more general one if its conditions are met.  However,
an equivalent monotonic formalisation is possible, though less concise.
This formulation is more-or-less equivalent to the interlingual solution 
described above, but
the advantage of locating this information in a transfer component is
that the monolingual lexicons 
are less complex and would not be affected by
any modifications which were only made when a translation
mismatch was noted.

The basic argument for semantic transfer is that our knowledge of
word meaning is very imperfect.  Minor discrepancies between the
meanings of the source and target language words will normally not even be noticed,
but occasionally contexts may arise where a subtle distinction becomes important.
If we try and make predicates language-independent, a discovery of a mismatch
could require extensive changes to the monolingual
lexicons. On the other hand, the use of
semantic transfer rules can be seen as an admission that a correspondence
may not be exact: we are not claiming that the denotations of the
predicates are identical, just that they are close enough for the contexts
we know about.  This level of indirection makes it easier to adjust
for discrepancies as they become known.  This may imply that semantic
transfer rules could only be satisfactorily formalised 
in a nonmonotonic system.

Transfer approaches are powerful enough to relate
arbitrary expressions in the source and target language semantics and 
because of this it is easy to
end up with an unmanageable set of transfer rules.  
Transfer rules are often used to attempt to fix problems that are really 
caused by idiosyncrasies in the target language, rather than a difference in meaning
between source and target language predicates.  
There is also a temptation to try and build rules that give a natural sounding
translation on a particular example, even though this may not generalise.
Later in this paper, we will give some examples where we think it is best to
assume that the monolingual components should do some of the work of
lexical choice and of blocking
bad translations. 

\subsection{Logical form equivalence and flat semantics}
\label{lfe}

Figure~\ref{vauq} and Figure~\ref{newvauq} are both somewhat misleading 
in the symmetry they imply between analysis and generation.  There are
 well-known theoretical problems in ensuring that a grammar can
generate from a particular semantic representation
which been discussed in the context of generation by
Shieber (1993) and in machine translation by Landsbergen (1987) and
Whitelock (1992), among others.    
One difficulty is the problem of logical form equivalence:
even though the grammar may accept a logical form (LF) 
logically equivalent to a particular LF which is input to the generator, there
is no guarantee that it will generate from that syntactic form
of the input LF.
To take a trivial example, an English grammar might naturally produce the
logical form in \exnum{+1}{a} from {\it fierce black cat}, while a
straightforward transfer from the natural Spanish representation of
{\it gato negro y feroz} shown in \exnum{+1}{b} would
produce the LF in \exnum{+1}{c}, which the English grammar probably
would not allow:
\begin{ex} 
\begin{subex}
$\lambda x [ \mbox{fierce}(x) \wedge (\mbox{black}(x) \wedge \mbox{cat}(x))  ]$ 
\end{subex}
\begin{subex}
$\lambda x [ \mbox{gato}(x) \wedge (\mbox{negro}(x) \wedge \mbox{feroz}(x)) ]$
\end{subex}
\begin{subex}
$\lambda x [ \mbox{cat}(x) \wedge (\mbox{black}(x) \wedge \mbox{fierce}(x)) ]$
\end{subex}
\end{ex}
One possible solution to this problem would be for
the generator to try all logically equivalent forms.  Unfortunately 
this is not practicable in general since the
logical form equivalence problem is undecidable even for first order
predicate calculus (Shieber 1993).  

Both Landsbergen's approach and the Shake-and-Bake framework for MT
were developed in response to this problem.  We have already suggested
that the difficulty of developing grammars which produce identical
representations makes the naive interlingual approach unsuitable for
Verbmobil.  Landsbergen's approach is even more restricting, since it
ensures identity of the interlingual representation by insisting that
the grammars for source and target language be isomorphic, which is
practically extremely difficult to achieve even for closely related
languages such as English and German (see e.g.~Beaven (1992) for the
difficulties of developing isomorphic grammars).  If, as we suggested
above, translation does not necessarily (or even usually) involve
preservation of denotation, this is clearly not the best approach.  We
will discuss Shake-and-Bake style approaches in \S\ref{tlinks}.


A related issue,
which is also addressed by
Shake-and-Bake, is the statement of phrasal 
transfer equivalences where material
may be interpolated between lexical items. 
For example, assuming the representation of adjectives shown above,
we cannot simply state
the following transfer rule for the equivalence 
English:{\it young bull} $\approx$ Spanish:{\it novillo}:
\[ \lambda x [ \mbox{young}(x) \wedge \mbox{bull}(x)) ] 
\leftrightarrow \lambda x [ \mbox{novillo}(x) ]\] 
Interpolated material is possible. For example, {\it young black bull} 
could have the following logical form:
\[ \lambda x [ \mbox{young}(x) \wedge (\mbox{negro}(x) \wedge \mbox{bull}(x)) ] \]
Either a more complex transfer rule has to be written which
allows for the intervening structure, or
the transfer component has to be able to rearrange the logical form, so that 
the rule does apply as written, which implies that the transfer component
needs knowledge of the logical properties of the connectives, and is
computationally expensive.
In general, the greater the structural complexity of the
representation, the worse the problem of constructing a LF acceptable
to the target grammar becomes.  
Copestake \etal \ (1995) discuss these issues in greater detail, and suggest
an approach to a
`flat' or minimally recursive
semantic representation which alleviates some of the problems.
We discuss this further below, in \S\ref{mrs}.

\section{Comparison of transfer formalisms}

\subsection{Criteria for comparison}
\label{criteria}

The
list of criteria below
is intended
as a general informal aid for comparison.
It is not complete; categories are interrelated and to some extent arbitrary, 
and there is some overlap between them.
There are tradeoffs between desirable features, and the weight
given to them will vary with the application.  
We have put an emphasis here on
features such as ease of acquisition,
modularity, and independence of rules, because we are concentrating
on the long term development of MT systems such as Verbmobil
and for this it essential that transfer rules be as easy to construct and
maintain as possible.
\begin{description}
\item[Coverage]  Are there some phenomena which cannot be handled
or can't be dealt with elegantly?  How important are they?

For theoretical purposes, this should be the main criterion.  A
transfer formalism should allow for any possible translation, just as a
grammatical formalism should be capable of dealing with any sentence of
the language.  But, as with grammar formalisms,
there are many caveats.  All 
systems can trivially deal with any specific translation by
simply stipulating the input/output pairing for that particular case.  
The is clearly appropriate for fixed conventional
phrases such as {\it auf Wiedersehen},
but it means that the criterion of coverage has to take into
consideration generality and elegance.  However, for a practical system, it may
not be worth expending effort
to treat some very rare construction.
\item[Modularity]  What assumptions are being made about the source and
target language grammars in the transfer component?

As we mentioned above,
some approaches to transfer essentially require that the source and
target grammars are developed in parallel.  This is undesirable in
principle, because it does not allow reuse of grammars developed for
other purposes, and also creates problems when extra languages are
added.  There is also the practical difficulty of managing grammar
development under these conditions, which would probably be insuperable
for Verbmobil since the English and German grammars are being developed
on different sites.  

None of the approaches discussed in detail here require isomorphism between
grammars or identity of LF for target and source sentences, but, at
least implicitly, they place some requirements for commonality
between the source and target language semantics.  It is an advantage
to minimise this if it distorts the grammar so that the
analysis becomes unnatural from a monolingual viewpoint.
\item[Independence]  Can transfer rules be written independently of
each other so that they nevertheless interact correctly?

Although this is not a topic that is discussed at
length in the MT literature, it appears that ensuring that
transfer rules interact correctly with each other and
with the monolingual grammars has been one of the main problems
in attempting to build robust MT systems and to scale-up systems 
to have reasonable coverage. 
Examples of where this might be problematic will be given later in the paper.
\item[Underspecification]  Is transfer between underspecified
representations possible?

The utility of this was discussed in \S\ref{underspec}.
\item[Generalisation]  Can useful generalisations be made to reduce
redundancy in transfer statements?   

We will give examples later in the
paper where generalisations about classes of
transfer rules are possible.
\item[Interaction with generation]
How flexible is the interaction with generation?

Some approaches to transfer, in particular Shake-and-Bake, cannot be
used with conventional generators.  Other approaches, which produce a 
logical form as the output of transfer, are more flexible.
The input to generation can be underspecified in various ways.
Some lexical choice is probably better left to the target
grammar and the generator --- prepositions are a fairly clear case
where it is might be possible to have a complete interlingua, but where 
the choice of preposition is very often idiosyncratic 
and lexically governed.  
For example, an adequate English grammar should make the correct distinction
between {\it in} and {\it on}, in order to generate {\it in the afternoon},
{\it on Thursday afternoon} but not {*\it on the  afternoon}, {*\it in 
Thursday afternoon}.  It is therefore unnecessary to attempt to do this in
transfer.  The prepositions {\it in} and {\it on}
have the same meaning in these examples, so the selection of
the actual preposition should be made
by the monolingual system.  Note the difference between this and the
example of {\it geben}$\approx${\it give}/{\it lend} discussed
in \S\ref{geben}. Transfer should be responsible
for the choice of {\it lend} rather than {\it give} in the context discussed,
because the sentence
{\it You gave me that book} could not be
excluded by the target grammar.

\item[Interaction with pragmatics and the domain model] 
How would an inference engine
interact with transfer?  

We will discuss this in very general terms in \S\ref{inference}, 
showing examples where inference would be needed to get the correct (or best)
translation.  The simplest method of interaction is to assume that the
inference component is resposible for selecting translations or
transfer rules from a range of possibilities provided by the transfer
component proper.  However there could be advantages if the formalism
supports a tighter interaction.  

The job of transfer is to
relate either lexical entries or language specific predicates in source
and target languages.  This process may be aided by the use of a domain
model which encodes real world knowledge
(at least for a restricted domain system such as Verbmobil) but
this is assumed to be language independent. 
We assume that the transfer component directly relates 
source and target representations rather than going via the domain
model, and thus the domain model is an aid to translation, rather than an
inherent part.  There are two main reasons for this:
\begin{enumerate}
\item It is implausible that any domain model for Verbmobil would be
able to cover all the situations that arise in the dialogues,
especially those which are reasons or excuses for disliking a
particular time for a meeting:
appointments at the dentist, sports and so on.  These situations are
outside the domain proper.  However, the system has to be able to
translate them, though a lower reliability is likely to be acceptable.
For crude translation at least, all that is needed is an equivalence
relationship between the lexical entries or lexical semantic structures.
\item The modularity of the system is improved if the domain model and the 
transfer module are distinct.  Changing domains should ideally involve
adding to the transfer rules and refining their conditions, rather than
completely rewriting them, as would be necessary if the domain model was central
(see the earlier discussion of knowledge-based MT). 
\end{enumerate}

\item[Implementation and control]  Simplicity and efficiency 
of efficiency are both clearly desirable.
We will not attempt to
give complexity results here, because it is unclear
whether they have much relationship to
the practical efficiency.  Any approach has the fundamental difficulty
that the number of results of transfer is potentially exponential: e.g.
if each of $n$ units (signs or semantic structures) has 
two transfer rules which may apply, there are $2^n$ possible results,
which is bad news for any realistic value of $n$.
However, there is a trade-off between 
efficiency and rule extensibility
if an attempt is made to control this in the transfer rules
directly.  Exactly the same arguments about declarativity apply as in
grammar rule development.  We will make some limited remarks on control in
\S\ref{control}. 
\item[Acquisition]  How easy is it to write transfer equivalences?
\end{description}

The aim with many recent approaches to transfer has been to achieve
declarativity and reversibility for the same reasons that it is now generally
accepted that grammatical formalisms should have these properties.  A
side effect of this is that it should be possible to evaluate transfer
formalisms abstractly, without considering details of the
implementation, just as it is possible to evaluate grammatical
formalisms without considering the particular parsing strategy chosen.
(It is, of course, important that an efficient implementation should be
possible in principle).  Similarly, concerns of modularity and expression
of generalisations apply as much to the expressions of translation
equivalence as they do to monolingual lexicons.



\subsection{Some approaches to semantic transfer}

In the next sections, we compare in detail three approaches to 
the statement of translation equivalences in semantically based
transfer.  These can be taken as representative of three styles,
according to the classification we gave in \S\ref{review}:
unification-based syntactic/semantic transfer,  
lexicalist transfer, and `pure' semantic transfer.
The specific systems we discuss are
the Verbmobil MDS system, the ACQUILEX tlink mechanism (Copestake, 1993)
and transfer
using minimal recursion semantics (MRS) (Copestake \etal\/\ 1995).  
The comparison is at a theoretical level of comparing the
formalisation of transfer equivalences, rather than comparing systems.
None of these approaches have been used in large scale MT so far.  In
some ways this is advantageous, since it appears that it is possible
(with sufficient effort) to make almost any MT approach work: the
resulting systems would be expected to differ considerably in ease of update
and maintenance and so forth, but this is extremely difficult to
evaluate.
It does seem to be generally true that MT systems are extremely
difficult to scale up, and our aim here in doing a largely 
theoretical study is to try and establish in advance where such
problems are likely to arise.

The MDS system is the only one which can be considered as
a full transfer component --- it has been used 
to construct translations of a number of real
dialogues collected for Verbmobil and is the only one of the three
approaches which has been used to treat an independently constructed
corpus, even though on a rather limited scale.
The ACQUILEX tlink mechanism was primarily intended as a technique for
representing multilingual lexical knowledge bases, rather than as an MT
system itself.  However, it is quite closely based on Shake-and-Bake,
which is currently being used to develop an MT system at Sharp Laboratories.
Tlinks themselves have been used by Trujillo (1995) in a
small MT system.
MRS is an approach to semantic representation designed to facilitate
transfer and generation, particularly with respect to the use of
underspecification and the problem of logical form equivalence
considered earlier.  It is currently being used for the semantics
of both English
and German grammars in the context of Verbmobil, although the German
(IBM Heidelberg)
version differs in some respects from the one we will consider here.
MRS would support forms of transfer other than the one sketched here,
which was intended mainly as a demonstration of the utility of MRS. 
In fact, as we will see, a transfer system might well be devised which
would take elements
from each approach.

All the approaches allow one to state equivalences between
semantic representations in the source and target language systems.
In very abstract terms, they all allow the statements
such as the following:
\[\mbox{\it schlecht}'(x) \transarrow \mbox{\it bad}'(x)\]
All assume an underlying typed constraint-based formalism,
and have a lexicalist orientation, though to varying degrees.
But they differ in the following basic ways:
\begin{enumerate}
\item What are the units 
to which the transfer rules apply?
\item How are the results of transfer combined?
\end{enumerate}

For the MDS the basic units of transfer are 
lexical signs, which are accessed via a procedure 
which operates on the parsed structure.
Combinations of lexical signs can be described,
but there are limitations on how straightforwardly these
can be stated.
The 
Verbmobil semantic construction operations are used
to combine the results
of transfer into a single DRS which can be input to generation.

Shake-and-Bake is based on the transfers of sets of lexical signs.
Arbitrary combinations of signs are possible, so the signs are 
thought of as forming a `bag' on which transfer operates to give a bag
of target signs.  
No logical form is
produced prior to the `generation' stage.  Instead the constraints 
are provided by the target bag of lexical signs, and generation can be
viewed as a process of sorting an unordered lexical input.  
The original algorithm for generation
was factorial in complexity but
Poznanski \etal\/\ (1995) describe
a polynomial algorithm, which relies on some 
(apparently quite natural) constraints
on the grammar.
The ACQUILEX tlink
formalism described in Copestake (1993) is 
a variant of the lexicalist approach which makes
it possible to state additional constraints on the 
syntactic relationship between the 
signs being transferred and to state generalisations about classes of
translation equivalence.

MRS transfer operates on 
arbitrary sets of semantic relations, with no access to the syntax.
Because the representation is flat, accessing individual elements
is trivial.  The target logical form is constructed by simply appending
the results of transfer.

\section{Transfer in the MDS}
\label{mds}

In this section we
discuss the transfer component which was implemented for the Verbmobil demonstrator.
This had to be designed to fit in with other parts of the Verbmobil system, which
somewhat restricted the possibilities.
The approach taken in the MDS can be characterised as a
variant of linked syntactic/semantic transfer.  The input to the transfer process
is the result of parsing a German sentence and instantiating its semantics,
and the transfer process walks over the HPSG parse tree, applying
transfer rules to the lexical signs.
All the rules are expressed in STUF.
The following is an example of one of the tree-walking rules:
\begin{verbatim}
tau(phrase_s &                     
    dtrs: (head_adj_struc &
           adj_dtr: #SemHead),#Args) => tau(#SemHead,#Args).
\end{verbatim}
Such rules apply to
the different types of syntactic structure in the parse tree (a head-adjunct
structure in this case), 
and recurse on the subcomponents.  Eventually the process bottoms out
at lexical signs:
\begin{verbatim}
tau(word_s & #Sign,#Args) => tau_lex(#Sign,#Args).

tau_lex(pred_name(#SourcePred) & #Sign,#Args) =>      
        tau_lex(#SourcePred,#TargetPred,#SemArgs,#Args,#Sign) &
        sem_lex(#TargetPred,#SemArgs).
\end{verbatim}
The effect of this general rule is to apply a specific transfer rule which will
instantiate the value of \verb+#TargetPred+ and \verb+#SemArgs+, which will enable
\verb+sem_lex+ to build the semantic structure for the transferred predicate.

A particular instance of a rule is given below:
\begin{verbatim}
tau_lex(schlecht,bad,[tau(#NP,#Args)],#Args,ad_syn(#NP)) => sem_t.
\end{verbatim}
This rule allows {\it schlecht} to be translated as
{\it bad} by giving
the correspondence between the German and the English
predicates and their respective arguments.  
The recursion \verb+tau(#NP,#Args)+ instantiates the arguments for 
\verb+sem_lex+.  
The fourth argument to \verb+tau_lex+, \verb+#Args+,
will be discussed below.
The sign as a whole has to unify with 
\verb+ad_syn(#NP)+ so this has the effect of
stipulating a syntactic condition
on the translation --- that {\it schlecht} is being used as an
adjective.  

To give an idea of how this rule is used in translation, consider the simple example
in \exnum{+1}{a}:
\begin{ex}
\begin{subex}
Das ist schlecht.
\end{subex}
\begin{subex}
That is bad.
\end{subex}
\end{ex}
The German semantic
structure for \exnum{0}{a} 
is as shown in Figure~\ref{mds1}.  
This structure is a DRS, but we have used the low-level representation
here because this makes the operation of the transfer rules clearer.
Of course, the transfer process sees more information than this
since it works on the full result of the parse, and thus has access to the syntactic 
structure as well as to the semantics.  We do not
show the full parse structure here, however, because it is complex 
and the details are not important, since the transfer rules
use high level predicates such as \verb+trans_syn+.
Thus the transfer rule writer should not need to know the details of the
syntactic representation, and a reasonable degree of modularity is maintained.
\begin{figure}
\begin{verbatim}
   [(sem:(sem_t &
          lambda:[] &
          drs:(drs_t &
               dom:[(A &
                     marker &
                     sort:statisch_c)] &
               conds:[(B &
                       supp_condition &
                       supp_drs:(drs_t &
                                 dom:[] &
                                 conds:[(alfa_condition &
                                         alfa_arg:(C &
                                                   marker &
                                                   snumber:individual) &
                                         alfa_type:demonstr &
                                         alfa_restr:(drs_t &
                                                     dom:[C] &
                                                     conds:[])),                       
                                        (dimen_condition &
                                         dimen_inst:C &
                                         dimen_pred:schlecht &
                                         dimension:quality)]) &
                       supp_state:A)]) &
          quants:[] &
          ip:(ip_t &
              cond:B &
              idx:A) &
          persp:(persp_t &
                 p_inst:A))]
\end{verbatim}
\caption{Semantics for {\it Das ist schlecht} omitting tense information}
\label{mds1}
\end{figure}


For simplicity we have also omitted
the representation of tense: 
the approach to tense is described in detail in Butt (1995).
In addition to the rule for
{\it schlecht} the transfer rules shown
below are used in the translation of \exnum{0}{}:
\begin{verbatim}
tau_lex(sein1,be1,[tau_tense(#Tense),tau(#O)],[],
        trans_syn(#S,#O & mod([#S]),#Tense)) => sem_t.

tau_lex(pronoun,pronoun,[#Marker,#AlfaType],[],
        sem:cond_sem(alfa_cond(#Marker,#AlfaType,[]))) => sem_t.
\end{verbatim}
The first rule is for predicative uses of {\it sein}.  Note the
translation of the
tense stipulated by \verb+tau_tense(#Tense)+.
Again, a syntactic condition is stipulated on the rule by
the final argument (which also instantiates \verb+#S+ and \verb+#O+).
The second
rule transfers pronouns: these are, in effect, analysed
using an interlingua, as the information about pronoun type and so on
is carried over directly from German
to English.

During actual translation, these rules will apply to an instantiated
structure, in such a way that the coindexation between the arguments of
the target language predicates is preserved on transfer to the source
language predicates. 
For example, the semantics for the
English predicate \verb+bad+ are specified as follows:
\begin{verbatim}
sem_lex(bad,#Args) => ad_dimen_sem1(bad,quality;mood_dim,#Args).
\end{verbatim}
The intermediate results 
after the application of the transfer rules 
for \exnum{0}{a} are shown in  Figure~\ref{mds2}.
\begin{figure}
\begin{verbatim}
  *c_id:c1027
   pred:alfa
   conc:_6375477
   m_id:m561
   args:[mod_rel:[]]
   mods:[type_info(alfa,demonstr)]
   
   *c_id:c1028
   pred:bad
   conc:_6376517
   m_id:m561
   args:[dimension: (mood_dim;quality)]
   mods:[]

  *m_id:m561
   sort:_6375311
   numb:individual
   gend:_6375313
   conc:_6375314
   c_id:_6375315
   mods:[]
   args:[c1028,c1027]
\end{verbatim}
\caption{Intermediate results of transfer (omitting tense)}
\label{mds2}
\end{figure}
These results are ``stitched together'' by
the semantic composition process.  
The English semantic structure
that results is shown in
Figure~\ref{mds3}. 
\begin{figure}
\begin{verbatim}
[(sem:(sem_t &
          lambda:[] &
          drs:(drs_t &
               dom:[(A &
                     marker &
                     sort:statisch_c)] &
               conds:[(B &
                       supp_condition &
                       supp_drs:(drs_t &
                                 dom:[] &
                                 conds:[(alfa_condition &
                                         alfa_arg:(C &
                                                   marker &
                                                   snumber:individual) &
                                         alfa_type:demonstr &
                                         alfa_restr:(drs_t &
                                                     dom:[C] &
                                                     conds:[])),
                                        (dimen_condition &
                                         dimen_inst:C &
                                         dimen_pred:bad & 
                                         dimension:(mood_dim ; quality))]) &
                       supp_state:A)]) &
          quants:[] &
          ip:(ip_t &
              cond:B &
              idx:A) & 
          persp:(persp_t &
                 p_inst:A))]
\end{verbatim}
\caption{Semantics for generating {\it That is bad} omitting tense information}
\label{mds3}
\end{figure}


Signs corresponding to arguments can 
straightforwardly be accessed by transfer rules operating on a verb etc,
making it easy to state rules which transfer source verb-argument combinations.
However, when it is necessary to apply a rule to a verb-adjunct combination, 
the treatment becomes more complex.
Consider the example in \exnum{+1}{a}.
\begin{ex}
\begin{subex}
Das pa\ss t schlecht bei mir
\end{subex}
\begin{subex}
That does not suit me at all
\end{subex}
\end{ex}
Here {\it passen} ({\it suit}) is analysed as an intransitive, with the 
experiencer being realised as an adjunct related to the verb by {\it bei}.
As a further complication, {\it schlecht} cannot be translated as {\it badly} here.
\footnote{As we will mention below,
this is actually due to a monolingual restriction, since {\it suit} is
a positive psychological predicate in English which does not normally
take a negative attitude denoting adverbial such as {\it badly}.}
The transfer component thus has to make {\it not at all} or {\it not well}
available as alternatives.
The rules used are shown below:
\begin{verbatim}
tau_lex(passen,suit,[tau_tense(#Tense), tau(#S), #OSem],[#OSem],
        intrans_syn(#S,#Tense)) => sem_t.

tau_lex(bei1,dummy_pred,[tau(#VP,[tau(#NP)|#Args])],#Args,
        prep_syn(#VP&sem:cond_sem(basic_cond(gehen2;
passen;kommen_in_frage,marker,list)),#NP)) => sem_t.

tau_lex(schlecht,negation,[focus_adv_sem1(at_all, grad, [tau(#VP,#Args)])],
[at_all|#Args],adv_syn(#VP&lex_pred(passen;passen1;gehen3;gehen2)))
=> sem_t. 
\end{verbatim}
Because {\it bei mir} is analysed as an adjunct, the 
translation of the NP is added to the list
\verb+#Args+, so that it will be available when the verb itself is translated.
Thus the fourth argument to the \verb+tau_lex+ rule for {\it passen}
is instantiated to a list containing the translation of {\it mir}, ensuring that
the correct coindexation will take place, so that this will form the 
object argument of the transitive verb {\it suit}.
No translation of {\it bei} itself is produced by this rule.
The rule is restricted so that it only applies in a limited range of cases: e.g. when the
verb is {\it gehen} (in the sense which can be translated as 
{\it be acceptable}) or {\it passen}. 
A rather similar treatment is used for {\it schlecht}, which adds 
\verb+at_all+ to
the value of \verb+Args+.

Finally, we should mention the interaction with the domain model.  
This is illustrated
below, where three alternative translations of {\it Termin} are
given:
\begin{verbatim}
tau_lex(termin,appointment,[],[],
  pred_sort(subsumes(veranstaltung_c))& #SourceSign) => tau_nn_nn(#SourceSign). 
tau_lex(termin,date,[],[],
  pred_sort(subsumes(zeit_punkt_c))& #SourceSign) => tau_nn_nn(#SourceSign). 
tau_lex(termin,appointment_slot,[],[],
  pred_sort(subsumes(zeit_periode_c))& #SourceSign) => tau_nn_nn(#SourceSign).
\end{verbatim}
The three usages are disambiguated by reference to the domain model concepts: 
\verb+veranstaltung_c+ (arrangement), \verb+zeit_punkt_c+ (time point),
\verb+zeit_periode_c+ (time period).

Considering the criteria we gave in the previous section, we can make the following 
observations:
\begin{description}
\item[Coverage] This may not be restricted in principle, but the solution that had
to be adopted to the example given above where {\it passen bei mir} was translated
as {\it suit me}, is somewhat messy.  This sort of transfer rule could be needed quite
frequently.
\item[Modularity] Even though syntactic information is accessed, this is achieved in
a reasonably abstract way, so the transfer rule writer does not
need detailed knowledge of the source and target grammars.  The use of the
semantic construction procedures means that there is no need to
directly tune the output of transfer to the target grammar.
\item[Independence] Transfer rules have to interact in quite complex ways in order
to ensure that the correct values are passed around.
\item[Underspecification] The approach would work with an underspecified semantic
representation of the type discussed in \S\ref{underspec}, but
as described it is operating on more fully specified representations.
\item[Generalisation] The formalism allows generalisations to be made over classes
of transfer rules, but as the rules are
currently expressed there is some redundancy.
\item[Interaction with generation] The output of transfer is a logical form, which could be
underspecified to allow lexical choice in generation, or 
even be reformulated by a strategic generator.
\item[Interaction with domain model etc]
As illustrated, the system can make the choice of transfer rule dependent on conditions
which are checked with respect to the domain model.
\item[Implementation and control] The tree-walking technique is efficient, but this is achieved at
the expense of flexible statement of some transfer relationships.
It is possible to state a variety of conditions which have
to be met for a rule to be applicable, which is very useful.  However
there is no clear distinction between such conditions and parts of the
rule which have to be evaluated in order to instantiate some variable.
Because the rules are written in STUF, evaluation order can be controlled: it is therefore possible
to have rules which act as defaults.  
\item[Acquisition] Many of the transfer rules are quite straightforward, but others could
only be written by someone with a quite detailed knowledge of the STUF formalism.
\end{description}

\section{Shake-and-Bake and tlinks}
\label{tlinks}

In this section, we discuss 
lexicalist approaches to transfer in some detail.
To start with we
give an introduction to
Shake-and-Bake (Whitelock, e.g.~1992)
which was the first approach in this category and then
describe the
ACQUILEX translation link (tlink) formalism, which was actually designed for
use in lexical knowledge bases (Sanfilippo \etal\/\ (1992),
Copestake (1993), Copestake \etal\/\ (in press)).


\subsection{Shake-and-Bake}

As originally described, Shake-and-Bake depends on
relating monolingual
lexical signs described within a lexicalist grammatical framework.  
The only information which is actually transferred
is the values of
the indices which become instantiated during
parsing (though see below).  The `generator'
is given a bag of lexical signs with their semantic arguments instantiated
and actually generates by parsing, trying all possible orderings, 
accepting only
those with the appropriate coindexation.  The advantage of this approach
is that the transfer component contains no information about the 
monolingual grammar, since it merely relates existing lexical entries, 
and that the problem of LF equivalence is to a large extent circumvented,
because the transfer component only indicates the relation of the lexical
signs by coindexation.  
One major disadvantage of Shake-and-Bake as originally described 
is lack of efficiency, since the
generator/parser has to consider a number of possibilities which is
factorial in the number of signs in the target sentence. 
However, Poznanski \etal\/\ (1995) 
show that polynomial complexity in Shake-and-Bake generation can
be achieved with some restrictions on the grammar.  

Consider the trivial example we used in the previous section:
\begin{ex}
\begin{subex}
Das ist schlecht
\end{subex}
\begin{subex}
That is bad
\end{subex}
\end{ex}
A sketch of the instantiated structure for \exnum{+1}{a} is 
given in Figure~\ref{tlink1}.
We have assumed a different treatment of predicative adjectives 
from that
in the MDS, in order to
make the treatment clearer.
\begin{figure}
{\small
\centering
\begin{avm}
  \[ phon & \< {\it das ist schlecht} \>\\
     sem & \@1 \[ pred & and \\
                  arg1 & \[ pred & deixis \\
                            arg & \@2 \] \\
                  arg2 & \@4 \[ pred & and \\
                                arg1 & \[ pred & schlecht\_state \\
                                      event & \@3 \\
                                      arg1 & \@2 \] \\
                                arg2 & \@5 \[ pred & present\\
                                              arg1 & \@3 \]\]\]\\
     dtrs & \[ \ldots &  \[ phon & \< {\it das} \>\\
                            sem & \@1 \]\\
               \ldots &  \[ phon & \< {\it schlecht} \>\\
                            sem & \@4 \]\\
               \ldots &  \[ phon & \< {\it ist} \>\\
                            sem & \@5 \]\]\]
\end{avm}

}
\caption{Schematic structure for {\it Das ist schlecht}}
\label{tlink1}
\end{figure}
The lexical signs, now with instantiated semantics, are treated as
forming a `bag', to which the lexical equivalences shown below
can be applied:
\begin{center}
{\small
\begin{avm}
\[ phon & \< {\it das} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & deixis \\
                            arg & \@2 \] \\
                  arg2 & \]\]
$\transarrow$
\[ phon & \< {\it that} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & deixis \\
                            arg & \@2 \] \\
                  arg2 & \]\]
\end{avm}

\begin{avm}
\[ phon & \< {\it schlecht} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & schlecht\_state \\
                            event & \@3 \\
                            arg1 & \@2 \] \\
                  arg2 & \]\]
$\transarrow$
\[ phon & \< {\it bad} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & bad\_state \\
                            event & \@3 \\
                            arg1 & \@2 \] \\
                  arg2 & \]\]
\end{avm}
\begin{avm}
\[ phon & \< {\it ist} \>\\
   sem & \[ pred & present\\
            arg1 & \@3 \]\]
$\transarrow$
\[ phon & \< {\it is} \>\\
   sem & \[ pred & present\\
            arg1 & \@3 \]\]
\end{avm}
}
\end{center}
The resulting bag of target lexical entries is shown below:
\begin{center}
{\small
\begin{avm}
\{\[ phon & \< {\it is} \>\\
   sem & \[ pred & present\\
            arg1 & \@3 \]\],
 \[ phon & \< {\it that} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & deixis \\
                            arg & \@2 \] \\
                  arg2 & \]\],
\[ phon & \< {\it bad} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & bad\_state \\
                            event & \@3 \\
                            arg1 & \@2 \] \\
                  arg2 & \]\]\}
\end{avm}}
\end{center}
These are then combined by the target grammar, 
a process which can be regarded as
a form of constraint resolution or of parsing with unordered lexical
items, rather than conventional generation from logical form.  
The result is the
structure shown in Figure~\ref{tlink3}: 
a logical form for the target sentence is produced,
but this is a side effect of producing the output string.
\begin{figure}
{\small
\centering
\begin{avm}
  \[ phon & \< {\it that is bad} \>\\
     sem & \@6 \[ pred & and \\
                  arg1 & \[ pred & deixis \\
                            arg & \@2 \] \\
                  arg2 & \@7 \[ pred & and \\
                                arg1 & \[ pred & bad\_state \\
                                      event & \@3 \\
                                      arg1 & \@2 \] \\
                                arg2 & \@8 \[ pred & present\\
                                              arg1 & \@3 \]\]\]\\
     dtrs & \[ \ldots &  \[ phon & \< {\it that} \>\\
                            sem & \@6 \]\\
               \ldots &  \[ phon & \< {\it bad} \>\\
                            sem & \@7 \]\\
               \ldots &  \[ phon & \< {\it is} \>\\
                            sem & \@8 \]\]\]
\end{avm}

}
\caption{Schematic structure for {\it Das ist schlecht}}
\label{tlink3}
\end{figure}

Note that, even though we used an interlingual representation
for {\it das} and {\it that}, it was
still necessary to explicitly specify the 
lexical equivalence, unlike in the MDS.
We have glossed over the treatment of morphology
but in Shake-and-Bake it is usual to assume an
inflectional morpheme 
which is simply treated as another lexical item 
(although there are other possible
approaches).

More complex cases involve relating sets of lexical items.  For example,
in order to represent the translation equivalence between {\it passen
bei NP}
and {\it suit NP} 
described in the previous section, and assuming the same analysis,
the following
relation could be used:
\begin{center}
\begin{avm}
\{\[ phon & \< {\it passen} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & passen \\
                            event & \@3 \\
                            arg1 & \@2 \] \\
                  arg2 & \]\],
\[ phon & \< {\it bei} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & bei \\
                            arg1 & \@3 \\
                            arg2 & \@1 \] \\
                  arg2 & \]\]\}
$\transarrow$
\end{avm}\hfill\mbox{}\\
\mbox{}\hfill
\begin{avm}
\[ phon & \< {\it suit} \>\\
   sem & \[ pred & and \\
                  arg1 & \[ pred & suit \\
                            event & \@3 \\
                            arg1 & \@2 \\
                            arg2 & \@1 \] \\
                  arg2 & \]\]
\end{avm}
\end{center}
The only relationship between the signs for {\it passen} and {\it bei}
is that indicated by coindexation of the event variable.

Now consider the more complex example which we discussed in the 
previous section:
\begin{ex}
\begin{subex}
Das pa\ss t schlecht bei mir
\end{subex}
\begin{subex}
That does not suit me well
\end{subex}
\end{ex}
We have changed the translation slightly for ease of exposition.
The Shake-and-Bake style of representing equivalences runs into some problems
in relating {\it schlecht} to {\it not well}, since {\it not}
and {\it well} cannot be related by coindexation in a conventional 
semantic representation.
In fact there is a general problem with 
Shake-and-Bake style mechanisms
which is that they cannot deal with any form of higher-order semantics.
For example, suppose the semantics of
\exnum{+1}{a} corresponds to the linearised representation shown in
\exnum{+1}{b}:
\begin{ex}
\begin{subex}
very aggressive dog
\end{subex}
\begin{subex}
$[x][\mbox{very}(\mbox{aggressive})(x)]$
\end{subex}
\end{ex}
In either case, the corresponding FS does not have a suitable index
with which to relate {\it very} and {\it sehr} because the argument is
a language specific predicate.  

Note that we cannot simply create a flat structure where {\it very} takes the
index $x$ as an argument, because this would prevent differentiation between
\exnum{+1}{a} and \exnum{+1}{b} because they would both have the representation
shown in \exnum{+1}{c}.
\begin{ex}
\begin{subex}
small very aggressive dog
\end{subex}
\begin{subex}
aggressive very small dog
\end{subex}
\begin{subex}
$[x][\mbox{small}(x) \wedge \mbox{very}(x) \wedge \mbox{aggressive}(x)]$
\end{subex}
\end{ex}

Since DRT is a first order theory, 
this particular problem is not going to arise in Verbmobil,
but the difficulty of representing the scope of negation
is a problem, as in the {\it schlecht}$\approx${\it not good}
example.  There are various ways round this,
one of which would be to give the
predicate itself an index which 
could be shared between SL and TL representations:
this would be analogous to the MRS handel 
mechanism discussed in \S\ref{mrs}.

\subsection{Tlinks}

The ACQUILEX translation link (tlink) language is a way of describing
lexical equivalences, which was intended to be utilisable by a variety
of approaches to MT.  Since the aim was to build a multilingual lexical
knowledge base (LKB) it was necessary to come up with a formalism that
directly related lexical entries, which resulted in a technique for
representing lexical equivalences which had close affinities with that
used in Shake-and-Bake.  Rather than concentrating on developing an
efficient transfer mechanism, the aim was to make the LKB
representation sufficiently flexible to be able to represent
generalisations about translation equivalences.  This involved a
combination of techniques:
\begin{enumerate}
\item Translation equivalences are themselves typed feature structures, so
generalisations can be made over classes of equivalence, making it
unnecessary to keep repeating the feature paths, for example.
This makes it easier to update the bilingual lexicon, if the structure
of a monolingual lexicon is changed.  For example, the {\it
schlecht}$\approx${\it bad} relationship could be encoded as follows:
\begin{verbatim}
schlecht_prd / bad_prd:
pred-adj-tlink.
\end{verbatim}
Here \verb+schlecht_prd+ and \verb+bad_prd+ 
refer to the entire lexical entries, which
then can instantiate the feature structure given by {\typef pred-adj-tlink}.
Even if the semantic representation of predicative adjective were changed,
all that is necessary is
to update the type {\typef pred-adj-tlink} for
this description to continue to apply.
\item A level of indirection was used 
so that the translation equivalences could always be related to
lexical entries.  This is shown diagrammatically for
the {\it schlecht}$\approx${\it bad} tlink in Figure~\ref{bad}, although
for a simple tlink, such as this one, the relationship between the
lexical entries and the forms to be linked is simply identity.
The indirection is significant for 
more complex relationship.  Consider a link between a singular and a
plural noun
(e.g.~{\it furniture}$\approx${\it M\"{o}bel}):
assuming that lexical entries are stored in the singular form, 
the relationship between the lexical entry and the target sign 
is simply the rule for plural formation.
This feature is also useful for phrasal tlinks such as 
that for {\it passen bei}$\approx${\it suit}.
The rule could simply be stated
as follows:
\begin{verbatim}
passen_1 bei_1 / suit_1 :
intrans-verb-adjunct-to-trans-tlink.
\end{verbatim}
The structure that this would expand to is shown diagrammatically in
Figure~\ref{passenbei}. 
\begin{figure}
\setlength{\unitlength}{0.9mm}
\begin{center}
\begin{picture}(125,25)(-5, 10)
\thinlines    \put(5,25){\makebox(0,0){SFS1}}
              \put(5,18){\makebox(0,0){schlecht}}
              \put(22,30){\makebox(0,0){identity}}
              \put(10,25){\vector(1,0){25}}
              \put(40,25){\makebox(0,0){SFS0}}
              \put(40,18){\makebox(0,0){schlecht}}
              \put(30,10){\makebox(0,0){$[ e,x ][ \mbox{schlecht\_state}(e,x) ]$}}
              \put(60,35){\makebox(0,0){\bf pred-adj-tlink}}
\thicklines
              \put(75,25){\vector(-1,0){30}}
              \put(45,25){\vector(1,0){30}}
\thinlines
              \put(80,25){\makebox(0,0){TFS0}}
              \put(80,18){\makebox(0,0){bad}}
              \put(90,10){\makebox(0,0){$[ e,x ][ \mbox{bad\_state}(e,x) ]$}}
              \put(97,30){\makebox(0,0){identity}}
              \put(110,25){\vector(-1,0){25}}
              \put(115,25){\makebox(0,0){TFS1}}
              \put(115,18){\makebox(0,0){bad}}
\end{picture}
\end{center}
\caption{Figurative representation of translation link relating {\it
schlecht} and {\it bad}}
\label{bad}
\end{figure}
\begin{figure}
\setlength{\unitlength}{0.9mm}
\begin{center}
\begin{picture}(125,50)(-5, -5)
\thinlines    \put(5,25){\makebox(0,0){SFS1}}
              \put(5,18){\makebox(0,0){passen}}
              \put(22,30){\makebox(0,0){includes}}
              \put(10,25){\vector(1,0){25}}

              \put(29,10){\makebox(0,0){includes}}
              \put(10,5){\vector(2,1){21}}
              \put(5,5){\makebox(0,0){SFS2}}
              \put(5,-2){\makebox(0,0){bei}}

              \put(40,25){\makebox(0,0){SFS0}}
              \put(40,18){\makebox(0,0){passen bei}}
              \put(60,37){\makebox(0,0){\bf intrans-verb-adjunct-to-trans-tlink}}
\thicklines
              \put(75,25){\vector(-1,0){30}}
              \put(45,25){\vector(1,0){30}}
\thinlines
              \put(80,25){\makebox(0,0){TFS0}}
              \put(80,18){\makebox(0,0){suit}}
              \put(97,30){\makebox(0,0){identity}}
              \put(110,25){\vector(-1,0){25}}
              \put(115,25){\makebox(0,0){TFS1}}
              \put(115,18){\makebox(0,0){suit}}
\end{picture}
\end{center}
\caption{Tlink relating {\it passen bei} and {\it suit}}
\label{passenbei}
\end{figure}
\item Although 
it is possible to represent the phrasal relationships as simply involving
sets of lexical signs, related only by coindexation, is is also possible
to express extra constraints on how the elements in the
bag can be combined.
However, doing this with full generality (while retaining the
advantages of the Shake-and-Bake mechanism with respect to the
logical form equivalence problem) comes at
the cost of introducing
an operation equivalent to LFG functional uncertainty (indicated by
{\tt includes} in Figure~\ref{passenbei})
and the author no longer considers
this a particularly promising approach.
\item 
The tlink mechanism allows rules as well as lexical entries 
to be related cross-linguistically, using {\it tlink-rules}.  For
example, causatives of movement verbs such as {\it fly} in English
({\it fly the kite}) can be related to an explicit causative
construction in the Romance languages (e.g.~{\it fece volare
l'acquilone}), see Copestake and Sanfilippo (1993).
This mechanism can also be used when
relating processes which are more directly equivalent in the two
languages.  
As a simple example, assume that 
in both the German and the English grammars,
predicative adjectives are related to
attributive ones by lexical rule.  A tlink rule relating
these lexical rules 
would have the result that the existence of 
a tlink between attributive adjectives
would automatically cause the predicative forms of those adjectives to be 
related as well, as indicated diagrammatically in
Figure~\ref{tlinkrule}.
\begin{figure}
\setlength{\unitlength}{1mm}
\begin{center}
\begin{picture}(140,75)(-20,-35)
\thinlines    \put(5,25){\makebox(0,0){SFS1}}
              \put(5,18){\makebox(0,0){schlecht\_att}}
              \put(10,25){\vector(1,0){25}}
              \put(40,25){\makebox(0,0){SFS0}}
              \put(40,18){\makebox(0,0){schlecht\_att}}
              \put(50,35){\makebox(0,0){\bf simple-tlink}}
\thicklines
              \put(55,25){\vector(-1,0){10}}
              \put(45,25){\vector(1,0){10}}
\thinlines
              \put(60,25){\makebox(0,0){TFS0}}
              \put(60,18){\makebox(0,0){bad\_att}}
              \put(90,25){\vector(-1,0){25}}
              \put(95,25){\makebox(0,0){TFS1}}
              \put(95,18){\makebox(0,0){bad\_att}}

              \put(5,15){\vector(0,-1){35}}
              \put(-10,0){\makebox(0,0){\shortstack{att/pred-\\ lexrule}}}
              \put(95,15){\vector(0,-1){35}}
              \put(110,0){\makebox(0,0){\shortstack{att/pred-\\ lexrule}}}

              \put(5,-25){\makebox(0,0){SFS1}}
              \put(5,-32){\makebox(0,0){schlecht\_prd}}
              \put(10,-25){\vector(1,0){25}}
              \put(40,-25){\makebox(0,0){SFS0}}
              \put(40,-32){\makebox(0,0){schlecht\_prd}}
              \put(50,-15){\makebox(0,0){\bf pred-adj-tlink}}
\thicklines
              \put(55,-25){\vector(-1,0){10}}
              \put(45,-25){\vector(1,0){10}}
\thinlines
              \put(60,-25){\makebox(0,0){TFS0}}
              \put(60,-32){\makebox(0,0){bad\_prd}}
              \put(90,-25){\vector(-1,0){25}}
              \put(95,-25){\makebox(0,0){TFS1}}
              \put(95,-32){\makebox(0,0){bad\_prd}}
\end{picture}
\end{center}
\caption{Tlink rule relating attributive and predicative adjectives,
shown instantiated with a tlink for {\it schlecht}$\approx${\it bad}}
\label{tlinkrule}
\end{figure}
Another case where tlink rules could be used
is the metonymic interpretation of {\it Stuttgart} as
{\it meeting in Stuttgart}, mentioned in \S\ref{underspec}.
Tlink
rules avoid the necessity to directly specify very large numbers of
predictable translation equivalences such as {\it fly
(causative)}$\approx${\it fare volare}, {\it run
(causative)}$\approx${\it fare correre}, {\it march
(causative)}$\approx${\it fare marciare} etc.  However, to use the
Shake-and-Bake algorithm, all these equivalences have to be compiled
out by the system, so there is no gain in efficiency of implementation.
\end{enumerate}

\subsection{Summary}

For further details of Shake-and-Bake and the 
tlink mechanism, see the papers cited.
Here we summarise the advantages and disadvantages, 
according to the criteria we discussed earlier:
\begin{description}
\item[Coverage] Some phenomena cannot be covered 
adequately without extensions to the formalism.  Shake-and-Bake
requires a lexicalist approach, since if semantic information is
introduced by grammar rules, it cannot be related by the lexical
equivalences.  This considerably restricts the class of grammars which
can be used, and even precludes using some versions of HPSG which
utilise the phrasal sort hierarchy to represent constructions
(e.g.~Sag, 1995).  Phrasal tlinks could be used, but the Poznanski
\etal\/\ generation algorithm is then not applicable.
\item[Modularity] 
Although syntactic conditions on transfer rules could be stated, the
formalism simply requires knowledge of the semantics.  The target
grammar can be developed independently from the set of transfer rules
because the logical form equivalence problem does not arise.
\item[Independence] 
If rules are fully instantiated, they are mutually independent.
However, the number of fully instantiated rules can become very large,
and if techniques such as tlink rules are used, interdependence between
rules has to be considered.
\item[Underspecification] 
The Shake-and-Bake formalism assumes that the output of transfer is a
set of lexical entries.  It is possible to relax this, so that
the output contains some underspecified lexical entries, but the efficiency
will be affected.
\item[Generalisation] A number of techniques for generalising over classes of 
transfer rules have been discussed, especially in the ACQUILEX tlink papers.
\item[Interaction with generation]  Standard approaches to generation
from a logical form cannot be used.  
There is no straightforward way of integrating
a strategic generation component.
\item[Interaction with domain model etc]
Interaction with a domain model which operated by specialising sorts in the
typed feature structure would be straightforward, but there are difficulties in
implementing more complex schemes, 
which are discussed in \S\ref{inference}, below.
\item[Implementation] 
Poznanski \etal\/\ (1995) describe a polynomial time Shake-and-Bake 
generator.  Most of the papers on Shake-and-Bake and related systems do not
mention control of application of transfer rules, but 
some control scheme is clearly required.
\item[Acquisition]  Coding translation equivalences in Shake-and-Bake
and in the ACQUILEX tlink mechanism is simpler than in the MDS.
Automatic acquisition of simple tlinks has been demonstrated
(Copestake \etal, in press) but the techniques described there
could be used with other approaches to transfer representation, especially
those that support generalisation.
\end{description}

\section{Transfer using Minimal Recursion Semantics (MRS)}
\label{mrs}

The description of MRS which follows is somewhat longer than 
the previous two sections, because we are describing both an approach to
semantic representation within a unification-based formalism and
how it can be utilised for transfer.
Much of the following discussion is taken from Copestake \etal\/\ (1995).
Briefly, minimal recursion semantics (MRS) is a framework 
for semantic representation within HPSG, which can be thought of as a 
meta-level language for describing semantic
structures.  It supports underspecification, 
so an MRS description will correspond to
a set of object language expressions.
For simplicity, in
the examples in this paper we will take the object language to
be predicate calculus, but MRS is intended
to be compatible with DRT.
We discussed the advantages of allowing underspecification in a
semantic representation for transfer in \S\ref{underspec}.
The other novel feature of MRS is that it is a flat semantic
representation.  In \S\ref{lfe} we discussed
the problems that arise when driving transfer from a conventional
semantic representation: MRS was developed in order to try and alleviate these
problems by devising a representation with no recursive structure.
Taking the example we discussed earlier, of {\it novillo}$\approx${\it young bull},
we mentioned that a simplistic encoding of this 
translation equivalence would not allow for
examples such as {\it young black bull} if this had the following LF:
\[ \lambda x [ \mbox{young}(x) \wedge (\mbox{negro}(x) \wedge \mbox{bull}(x)) ] \]
In contrast,
a flat semantic representation might be:
\[ \lambda x \bigwedge[{\mbox{young}(x), \mbox{negro}(x), \mbox{bull}(x)} ] \] 
A transfer rule can now be written without having to take account of the possible
bracketings.  For further discussion, see Trujillo (1995).
The above example is trivial, of course, but in \S\ref{mrstrans}, 
we will show how flat semantics makes it easier to write some more complex transfer rules.

\subsection{Introduction to MRS}

Assuming that
it is desirable to reduce the use of recursive structures, it is necessary
to find an alternative method of representing scope.
For example, ignoring tense for the moment, consider the representation
of: 
\begin{ex}
Every tall man is old
\end{ex}
In this case there is only one possible scope for 
{\it every} which is shown in \exnum{+1}{}
using generalized quantifiers:
\begin{ex}
$every(x, man(x) \wedge tall(x), old(x))$ 
\end{ex}
We should, therefore, be able to retrieve this reading unambiguously
from the semantic representation that the grammar constructs for this
sentence.  However, if we have the totally flat structure shown 
in \exnum{+1}{}
it is impossible to retrieve the correct reading unambiguously, because
we would get the same structure for \exnum{+2}{}.
\begin{ex}
$\bigwedge\{every(x), man(x), old(x), tall(x)\}$ 
\end{ex}
\begin{ex}
Every old man is tall
\end{ex}
A flat representation is required which preserves
sufficient information about the scope of a quantifier to be able
to construct all and only the possible readings for the sentence. 
Similar remarks apply with respect to the representation of the scope
of {\it not}, {\it or} and so on.

This effect can be achieved while retaining the flatness of
the representation by adding extra variables, 
which have the effect of capturing scope information.  For
example, we could represent \exnum{0}{} as \exnum{+1}{}:
\begin{ex}
$\bigwedge\{\mbox{every}(x, 1, 2), \mbox{man}_1(x), \mbox{old}_1(x), 
\mbox{tall}_2(x)\}$
\end{ex}
These extra variables can be thought of as handles which enable us to
`grab' particular propositions in the flat list (cf the use of {\it labels}
in Frank and Reyle, 1994).
In \exnum{0}{} the scoped representation can be reconstructed by replacing the
handles in the restriction and body arguments of {\it every} with the
propositions tagged by those handles.  From now on we will drop the
use of $\wedge$ and assume implicit conjunction.

In the example above, everything which would
have been inside a set of braces in a conventional formula 
was tagged with the
same subscript.  Nested quantifiers do not require multiple indices
on a single conjunct, since
we can trace the nesting via the restriction of the embedded quantifier.
The reason why this approach is not just a notational variant of 
a standard representation is that the handles can be underspecified
to represent multiple scopes.
For example, the underspecified representation of \exnum{+1}{a}
would be \exnum{+1}{b}:\footnote{We are
oversimplifying somewhat here, since in order to be able to
represent any information about the relative scope of more than
two quantifiers, it is necessary to represent the relative scope of
pairs of quantifiers (Frank and Reyle, 1994). This can be accommodated
in the MRS representation, but we do not give the details
since there are not relevant here.}
\begin{ex}
\begin{subex}
Every dog chased some cat.
\end{subex}
\begin{subex}
$\mbox{every}_1(x, 3, n), \mbox{dog}_3(x), \mbox{cat}_7(y), 
\mbox{some}_5(y, 7, m), \mbox{chase}_4(e,x,y)$
\end{subex}
\end{ex}
Here $n$ and $m$ stand for variables over handles.  The scoped representations
would be:
\begin{ex}
\begin{subex}
$\mbox{every}_1(x, 3, 4), \mbox{dog}_3(x), \mbox{cat}_7(y), 
\mbox{some}_5(y, 7, 1), \mbox{chase}_4(e,x,y)$\\
(wide scope {\it some})
\end{subex}
\begin{subex}
$\mbox{every}_1(x, 3, 5), \mbox{dog}_3(x), \mbox{cat}_7(y), 
\mbox{some}_5(y, 7, 4), \mbox{chase}_4(e,x,y)$\\
(wide scope {\it every})
\end{subex}
\end{ex}

MRS proper is defined in terms of feature structures (FSs), 
rather than the linearized representation shown above.  
The semantic representation has two parts, {\sc content} and {\sc context}, as
usual in HPSG, but
here we are mainly concerned with the {\sc content} value.
An MRS expression 
consists of
a structure of type {\typef mrs-struct}, 
with appropriate features {\sc handel} 
and {\sc liszt}, which take values of type {\typef handle} 
and {\typef list} respectively.  
The feature name
{\sc liszt} is used to distinguish the 
non-recursive semantic structure from ordinary lists: 
the values of {\sc liszt}s have to be treated like sets in some
respects as we will see below.
{\sc handel} is used for the other main feature of the compositional semantics.
We adopt the normal convention of writing feature names in (small)
capitals and types/sorts in italics.
In what follows, we will often use liszt and handel in 
the normal lower case font to refer
to the values of those features. 

The sort {\typef mrs-struct} also has the  
appropriate feature {\sc index} which plays much the same role 
as a lambda variable in conventional representations.
The value of
{\sc liszt} is defined to be a flat list of {\typef rel}s
(relations), which all have
{\sc handel}s and other features depending on their sort.
As in Pollard and Sag (1994:chap.~8, sec.~5), 
the actual relation is indicated by the
sort of the rel.
Determiners, such as {\it every}, have rels with appropriate features 
{\sc bv} (bound variable) which takes a value of sort {\typef ref-ind},
and {\sc restr} (restriction) which takes a {\typef handle}.
MRS assumes a neo-Davidsonian style of representation
with explicit event variables. 
Verb rels have a feature {\sc event} which takes 
an event variable.  
They also
have features such as {\sc act} (actor) and {\sc und}
(undergoer) following Davis (forthcoming).
Common nouns have rels with the feature {\sc inst} which
takes a {\typef ref-ind}. 
An example of the (unscoped) MRS representation for
\exnum{-1}{a} is shown in Figure~\ref{unsc-chasing}.
Here the variable sorts and the internal
structure of the indices are not shown, but 
only the coindexation between them, indicated as usual 
by boxed integers.
\begin{figure}
\begin{center}
{\small
\begin{avm}
  \[ handel & \@1 $\vee$ \@5 \\
    index & \@8\\
    \avmspan{liszt \;
    \<  \osort{every\_rel}
       {\[ handel & \@1\\
           bv & \@2\\
           restriction & \@3 \]},
       \sort{dog\_rel}
       {\[ handel & \@3\\
           inst & \@2 \]},
       \sort{some\_rel}
       {\[ handel & \@5\\
           bv     & \@6\\
           restriction & \@7 \]}, \avmr}\\ 
     \avmspan{\hspace{0.3in}\;\avml
     \sort{cat\_rel}
       {\[ handel & \@7\\
           inst & \@6 \]},
     \sort{chase\_rel}
      {\[ handel & \@4\\
          event & \@8\\
          act & \@2\\
          und & \@6 \]}\>}\]
\end{avm}}
\end{center}
\caption{Unscoped representation for {\it every dog chased some cat}}
\label{unsc-chasing}
\end{figure}
The handel shown at the outer level allows the sentence to be
embedded, as in {\it Sandy said that every dog chased some cat}, for
example.  Here it is a disjunct of the handels
of the quantifiers, because no scope has been assigned: in the scoped
representation the handel will be the handel of the widest scoped quantifier.
Scoped quantifiers have to be represented as having both 
a restriction and a body.  However, 
for the underspecified representation the body
of the quantifiers is left unspecified.
The representation can be monotonically enriched to either 
scoped structure by appropriate coindexation
of the handels and instantiation of the {\sc body}\/s of the
quantifiers.
One of the scoped representations (wide scope {\it some}) 
is shown in Figure~\ref{sc-chasing}.
Further details are given in Copestake \etal\/\ (1995).
\begin{figure}
\begin{center}
{\small
\begin{avm}
  \[ handel & \@5 \\
    index & \@8\\
    \avmspan{liszt \;
    \<  \osort{every\_rel}
       {\[ handel & \@1\\
           bv & \@2\\
           restriction & \@3 \\
           body & \@4\]},
       \sort{dog\_rel}
       {\[ handel & \@3\\
           inst & \@2 \]},
       \sort{some\_rel}
       {\[ handel & \@5\\
           bv     & \@6\\
           restriction & \@7 \\
           body & \@{1}\]}, \avmr}\\ 
     \avmspan{\hspace{0.3in}\;\avml
     \sort{cat\_rel}
       {\[ handel & \@7\\
           inst & \@6 \]},
     \sort{chase\_rel}
      {\[ handel & \@4\\
          event & \@8\\
          act & \@2\\
          und & \@6 \]}\>}\]
\end{avm}}
\end{center}
\caption{MRS for {\it every dog chased some cat} with {\it some} taking
wide scope}
\label{sc-chasing}
\end{figure}

From now on, rather than simply using boxed integers to indicate
reentrancy in the conventional way, we will indicate their types for clarity,
using h for handel, e for event, x for entity, and i for individual 
(event or entity).
For convenience, arguments to rels are shown as atomic entities in this
document, but indices actually have a complex structure as in Pollard
and Sag (1994).
The MRS representation of \exnum{+1}{} is shown in Figure~\ref{suit}.
\begin{ex}
That really doesn't suit me well
\end{ex}

Here the handels of rels which are not scoped with
respect to one another are unified: in general
whenever two {\typef mrs-struct}s are combined, 
the handels are unified to create the handel for the result unless 
a rel in the liszt of one {\typef mrs-struct}
takes the handel of the other as an argument. 
Because scope has not been assigned, the outermost handel is not
coindexed with any of the handels internal to the rels --- it should be
thought of as a disjunction of h2 and h10.
The new liszt is
constructed by appending the liszts of the 
structures.
\begin{figure}
\begin{center}
{\small
\begin{avm}
  \[ handel & \@{h1}\\
    index & \@{e8}\\
    \avmspan{liszt \;
    \<  \osort{good\_rel}
       {\[ handel & \@{h4}\\
           arg & \@{e8} \]},
       \sort{neg\_rel}
       {\[ handel & \@{h3}\\
           inst & \@{h4} \]},
       \sort{suit\_rel}
       {\[ handel & \@{h5}\\
           event     & \@{e8}\\
           act & \@{i7}\\
           und & \@{x6} \]}, \avmr}\\ 
     \avmspan{\hspace{0.3in}\;\avml
     \sort{ego\_rel}
       {\[ handel & \@{h9}\\
           inst & \@{x6} \]},
     \sort{deixis\_rel}
      {\[ handel & \@{h10}\\
          inst & \@{i7} \]},
     \sort{real\_rel}
      {\[ handel & \@{h2}\\
          arg & \@{h3} \]}
\>}\]
\end{avm}}
\end{center}
\caption{MRS for {\it That really doesn't suit me well}}
\label{suit}
\end{figure}

The inclusion of the 
unresolved {\typef ego\_rel} and {\typef deixis\_rel} 
reflects the idea that the
semantics should preserve enough surface information to facilitate
translation for cases where there is a straightforward mapping between
languages. 
This is not built into MRS in any way, however, so the representation
is more flexible than QLF or Shake-and-Bake.
Note that {\it
well} has {\typef good\_rel} in its semantics and that {\it really}
contains a {\typef real\_rel}.  
For adverbs where there is a systematic distinction in meaning from the
corresponding adjective 
it would be necessary to use a separate rel.  The current treatment
does not preclude {\it suit}, {\it real} and so on from being
polysemous in that there could be subsorts of {\typef suit\_rel}, 
{\typef real\_rel} and so on.   
As before we omit any representation of tense.  Note that both {\typef
real\_rel} and {\typef neg\_rel} take handels as arguments to allow for
the difference between
\exnum{+1}{a} and \exnum{+1}{b}:
\begin{ex}
\begin{subex}
That really doesn't suit me
\end{subex}
\begin{subex}
That doesn't really suit me
\end{subex}
\end{ex}
This contrasts with PPs such as {\it on Monday} and {\it in Foothill
Park} which are represented as taking events to avoid spurious
ambiguity. 

One very general assumption is that this approach is 
strongly but not completely lexicalist --- the vast majority of rels
will be introduced lexically (i.e.~in lexical entries or lexical rules). 
The main exception to this are relations indicating
the type of speech act, {\typef y\_n\_q\_rel}, {\typef imp\_rel} etc which
are introduced syncategorematically.
There will normally
be a distinct rel for each sense of a word.  
Rel names usually indicate the orthography of the corresponding lexical
item (e.g.~{\typef would\_rel}).
Senses are distinguished by words, not numbers.
For example, {\typef work\_acceptable\_rel} corresponds to the sense of
{\it work} in \exnum{+1}{}:
\begin{ex}
Does that work for you too?
\end{ex}
Rels are not distinguished purely on the basis of syntax: thus
{\typef eat\_rel} would cover both transitive and intransitive forms,
the latter having an understood undergoer.
Some rels are not of this form: for example {\typef ego\_rel}
corresponds to {\it I} or {\it me}, {\typef deixis\_rel} to {\it that},
{\typef neg\_rel} indicates negation
and {\typef y\_n\_q\_rel} and {\typef imp\_rel} stand for yes-no-question
and imperative respectively.

Because rels are sorts, they will be in a hierarchy.  This can be
utilised in order to make the appropriate linguistic generalisations:
for example, {\typef come\_move\_rel} could be a subsort of {\typef move\_rel},
which would, among other things, allow a general expression of the
alternations that apply to movement verbs.  Note that this
hierarchy will not usually correspond directly to the domain model
concept hierarchy, since this is motivated by the real world rather than linguistic
considerations.


\subsection{Translation}
\label{mrstrans}

Translation between MRS representations depends on setting up
the correct reentrancies between the arguments of the
rels, as with the previous systems we 
discussed.\footnote{For simplicity, we show coindexation between 
the complete indices here, but the 
representation has to be slightly more complex, since in HPSG indices contain 
information which will not be shared between
languages, such as gender.}
As before, we initially show the example 
of translation of {\it das ist schlecht}, using a simplified semantics
(Figure~\ref{easytrans}).
As with tlinks, transfer
equivalences can themselves be seen as feature structures:
reentrancies are used between the 
two halves of the equivalences
to indicate argument identification and 
generalizations about classes of transfer equivalence are specified using
sorts.
Note that the output MRS is underspecified, in that the top-level
{\sc handel} value is not instantiated, but 
constraints on the English grammar would mean that
h1 is the only possible value.
\begin{figure}
{\small
\begin{tabular}{ll}
Input sentence: & Das ist schlecht\\[0.2in]
Input MRS: & 
\begin{avm}
  \[ handel & \@{h1}\\
    \avmspan{liszt \;
    \<  \osort{deixis\_rel}
       {\[ handel & \@{h1}\\
           inst & \@{x2}\]},
        \sort{schlecht\_rel}
       {\[ handel & \@{h1}\\
           arg & \@{x2} \]} \>} \] 
\end{avm}\\[0.2in]

Transfer equivalences: &
\begin{avm}
\sort{deixis\_rel}
{\[  handel & \@{h2}\\
     inst & \@{x1} \]}

$\transarrow$

\sort{deixis\_rel}
{\[  handel & \@{h2}\\
     inst & \@{x1} \]}
\end{avm} \\

&
\begin{avm}
\sort{schlecht\_rel}
{\[  handel & \@{h3}\\
     arg & \@{x4} \]}

$\transarrow$

\sort{bad\_rel}
{\[  handel & \@{h3}\\
     arg & \@{x4} \]}

\end{avm} \\[0.2in]

Instantiated equivalences: &
\begin{avm}
\sort{deixis\_rel}
{\[  handel & \@{h1}\\
     inst & \@{x2} \]}

$\transarrow$

\sort{deixis\_rel}
{\[  handel & \@{h1}\\
     inst & \@{x2} \]}
\end{avm} \\

&
\begin{avm}
\sort{schlecht\_rel}
{\[  handel & \@{h1}\\
     arg & \@{x2} \]}

$\transarrow$

\sort{bad\_rel}
{\[  handel & \@{h1}\\
     arg & \@{x2} \]}

\end{avm} \\[0.2in]

Output MRS:  &
\begin{avm}
  \[ handel & \@{h1}\\
    \avmspan{liszt \;
    \<  \osort{deixis\_rel}
       {\[ handel & \@{h1}\\
           inst & \@{x2}\]},
        \sort{bad\_rel}
       {\[ handel & \@{h1}\\
           arg & \@{x2} \]} \>} \] 
\end{avm}\\[0.2in]
Output sentence:  & That is bad
\end{tabular}}
\caption{Transfer using MRS}
\label{easytrans}
\end{figure}

Most pure semantic transfer systems require {\it structural} transfer
rules which handle the reordering 
and rearrangement of the semantic structures.  
Structural transfer rules are
minimized by the MRS representation, which is desirable not only
because of the reduction in number and
complexity of the transfer rules, but
because it is extremely difficult to guarantee that phrasal transfer rules and
structural rules will interact properly.
Transfer between two MRS representations essentially requires a
single structural transfer rule, if we think of
the value of {\sc liszt} as a set of rels:
\begin{definition}[Structural transfer rule] The translation $\tau$ of
a set of rels $z= x \cup y$ is defined as
\[\tau(x \cup y) = \tau_l(x) \cup \tau(y)\]
where $\tau_l$ is the base case:
$x$ translates as $\tau_l(x)$ iff there is some 
transfer equivalence
such that the input unifies with $x$ giving $\tau_l(x)$ as the output. 
\end{definition}
In other words, the translation of a {\typef mrs-struct} is the 
translation of some subset of the liszt unioned
with the translation of the remainder of the 
liszt.\footnote{Although this rule
could be regarded simply as a constraint between
input and output structures, it has to be implemented 
specially within constraint based formalisms which lack a
representation of sets.}

\subsection{Transfer equivalences}

All transfer equivalences consist of relationships between sets of rels but
the vast majority will be lexically motivated.
Some rels such as {\typef neg\_rel},
{\typef ego\_rel} and {\typef deixis\_rel} 
are not language-specific: in general we allow
relations with a sort which
is defined to be interlingual 
to simply be transferred as they stand between source and target.
In general, however, transfer relationships have to be explicitly
stipulated.
For example, below we give
examples of transfer equivalences that could apply in
the translation of \exnum{+1}{a} to \exnum{-2}{} (repeated as
\exnum{+1}{b}):
\begin{ex}
\begin{subex}
das pa\ss t schlecht bei mir
\end{subex}
\begin{subex}
that doesn't suit me well
\end{subex}
\end{ex}
\begin{figure}
\begin{center}
{\small
\begin{avm}
  \[ handel & \@{h1}\\
    index & \@{e8}\\
    \avmspan{liszt \;
    \<  \osort{schlecht\_rel}
       {\[ handel & \@{h3}\\
           arg & \@{e8} \]},
       \sort{passen\_rel}
       {\[ handel & \@{h5}\\
           event     & \@{e8}\\
           theme & \@{i7}\]}, 
       \sort{bei\_rel}
       {\[ handel & \@{h5}\\
           arg1     & \@{e8}\\
           arg2 & \@{x6}\]},\avmr}\\ 
     \avmspan{\hspace{0.3in}\;\avml
     \sort{ego\_rel}
       {\[ handel & \@{h9}\\
           inst & \@{x6} \]},
     \sort{deixis\_rel}
      {\[ handel & \@{h10}\\
          inst & \@{i7} \]},
     \sort{echt\_rel}
      {\[ handel & \@{h2}\\
          arg & \@{h3} \]}
\>}\]
\end{avm}}
\end{center}
\caption{MRS for {\it Das pa\ss t echt schlecht bei mir}}
\label{passen}
\end{figure}



\begin{center}
{\small\singlespace

\begin{avm}
\< \sort{echt\_rel}
{ \[  arg & \@{h1} \]} \>

$\transarrow$

\<\sort{real\_rel}
{\[  arg & \@{h1} \]} \> 
\end{avm}\\[0.2in]
\begin{avm}
\< \sort{passen\_rel}
{\[ event & \@{e3}\\
    theme & \@{i1} \]}, 

\sort{bei\_rel}
 {\[ arg1 & \@{e3}\\
     arg2 & \@{x2} \]}\>

$\transarrow$

\< \sort{suit\_rel}
{\[  act & \@{i1}\\
     und & \@{x2} \]} \>
\end{avm}\\[0.2in]
\begin{avm}

\< \sort{schlecht\_rel}
{\[ arg & \@{e1} event \]} \>

$\transarrow$

\< \sort{neg\_rel}
{\[ arg & \@{h2} \]}, 

\sort{good\_rel}
{\[ handel & \@{h2} \\
    arg & \@{e1} \]} \>

\end{avm}}
\end{center}
Note that we have refrained from relating the non-argument
handels and event variables
explicitly here.  It is redundant to repeat the information that the
handels and events are in one-to-one relationship for each rel, since
it could simply be inherited from a general sort for transfer
equivalences (in a manner comparable to
the use of tlinks).
The rule would apply to both adjectival and adverbial uses
of {\it schlecht} (we assume that {\it good} and {\it well} both have
{\typef good\_rel} in their semantics).  However, we have specialized the
antecedent so that it only applies when an event is modified.

Note that the use of handels allows a transfer relationship
to be expressed by relating variables in a way which would be impossible
with a more conventional semantic representation without adopting additional
devices (such as transfer variables).
In many cases, Shake-and-Bake transfer rules
could be generated automatically from MRS rules by unifying the MRS
rels with their corresponding lexical entries.  MRS 
transfer is more flexible in that it
does not require that there be corresponding lexical entries, thus it will
work with grammar rules that introduce semantic information, such as
{\typef y\_n\_q\_rel}.  
Expressing the relationship between rels rather than signs
makes a more elegant treatment of some cases of translation
mismatch possible:
see the comparison in Copestake \etal\/\ 1995.
That paper also
discusses how the mechanisms for generalisation which were developed for tlinks in ACQUILEX
have straightforward counterparts within MRS.

\begin{description}
\item[Coverage] At present, there are no obvious limitations, except those which are
inherent to MRS itself (lack of a general treatment of higher-order constructions).
This is irrelevant if the underlying representation is assumed to be DRT.
\item[Modularity] There is no explicit reference to the
syntactic structure of the source or target languages.
The logical form equivalence problem is mitigated in
a way comparable to that in Shake-and-Bake.
\item[Independence] The use of partially specified
transfer rules discussed in Copestake \etal\/\ (1995)
would make complex interactions possible, but otherwise rules are independent of
one another.
\item[Underspecification] Various types of underspecification are possible in MRS
and could straightforwardly be maintained in transfer.
\item[Generalisation] There are similar possibilities to those for tlinks.
This is discussed in Copestake \etal\/\ (1995).
\item[Interaction with generation] Transfer produces a logical form, which
could be underspecified, requiring the generation component to
make some lexical choices etc.  It would also be
possible for a strategic generation component to rearrange the logical form. 
\item[Interaction with domain model etc] See the remarks in the next section. 
\item[Implementation] The transfer process itself can be implemented in
a rather similar way to the transfer between bags of signs used in
Shake-and-Bake.  
Partially specified transfer rules would be more difficult to
implement efficiently and correctly.  
\item[Acquisition]  MRS links are more abstract than tlinks and might be more
difficult to write because of that, especially if the naming conventions for
rels are unintuitive.
But automatic acquisition techniques
developed for tlinks should work 
equally well for MRS links.
\end{description}


\section{Inference and interaction with generation}
\label{inference}

In the previous sections, we have discussed examples of translation
equivalence which were constrained by parts of the
logical form.  However, many translation choices require more general
inference mechanisms, which have to be implemented externally to the
transfer mechanisms outlined above.  These might involve 
relatively open ended reasoning using a
dialogue model or a domain model, in contrast with the more limited
possibilities for inference available within a typed unification system.
We are not concerned here with
exactly how such inference should be implemented but only with the
interaction with transfer.   
In principle there are at least three options:
\begin{enumerate}
\item The transfer equivalence statement could require
information from the domain/dialogue model.

The {\it Termin} example given in \S\ref{mds} illustrates this approach.
\item Transfer could overgenerate, leaving generation to perform a
semantic well-formedness check on the target LF
to exclude those which are contextually impossible or implausible.
This alternative is
not attractive with a straightforward version of the lexicalist
approach, because the full LF for the target is not produced prior to
generation.  The filtering mechanism would thus have to operate after
generating the complete output sentence, and since this is the most
computationally expensive step this is not an attractive option.
\item Transfer could generate a semantically correct target LF, which
was reformulated by a strategic generation component in order to improve the
output sentence.  In general such reformulation will involve some
inference.  Similar remarks about the unsuitability of this approach
for the lexicalist mechanism apply as above.
\end{enumerate}
The first two options allow selection between
translations --- the third allows reformulation of a semantically 
plausible LF.
There is also the possibility 
that transfer would overgenerate but that the target grammar would filter
out some possibilities.  This might be appropriate to block the
generation of
{?\it that suits me badly}, for example, as we will discuss in
\S\ref{control}. 
This is not a case of inference in the same
way as the other options listed above, but we mention it here for 
completeness.

\subsection{Inference for choice of translation equivalence}

The most straightforward examples of a requirement for inference
is where there are multiple possible translations of a word which
cannot be distinguished by immediate context, but only by reasoning
about the state of the dialogue etc.  For example, 
as we mentioned in \S\ref{mds}, {\it Termin} in
German can be translated as {\it date} or {\it
appointment} in the Verbmobil 
domain.\footnote{The situation is actually more complex than
this, since neither translation is good in some contexts, where {\it
meeting} would be preferable.  We discuss this further in \S\ref{control}.}
The following examples illustrate this from the Verbmobil
dialogues:  
\begin{ex}
\begin{subex}
Das ist schlecht - da habe ich um 14.30 einen Termin beim
Zahnarzt.\\
That's bad - I have a dentist's appointment at 14:30.
\end{subex}
\begin{subex}
Ja ich habe schon davon geh\"{o}rt und habe den Termin noch freigehalten.\\
Yes, I've already heard about that and have reserved that date.
\end{subex}
\end{ex}

There appears to be a fundamental distinction
between the {\it date} and {\it appointment} 
usages --- the first is describing a slot and the second a filler.  
Getting this wrong in translation will normally be a serious error.
For example:
\begin{ex}
That's bad - I have a date at the dentist at 14:30.
\end{ex}
The difficulty here is partly that, although English also has both
usages for {\it date}, it is restricted in the filler sense to
social events, usually romantic or sexual encounters. 
But although there may sometimes be local clues about the translation, the
reasoning involved may be quite complex and depend on a knowledge of
the context of the source sentence.
This is a very clear example where the domain model should be used
to resolve the translation, because it allows the system to take
advantage of the restricted topics of the dialogue.

Neither the tlink nor the MRS approach to transfer have mechanisms
for accessing the domain model within the formalism.  It would,
of course, be possible to simply add an arbitrary condition
statement to the expression of translation equivalences in either 
formalism. 
\begin{quote} 
\begin{avm}
\sort{termin\_rel}
{\[  inst & \@{x1} \]}

\;$\Longrightarrow$

\sort{date\_rel}
{\[  inst & \@{x1} \]}
\end{avm} \\[0.2in]
Condition: veranstaltung\_c (x1)
\end{quote}

The alternative technique mentioned above would decouple the two
mechanisms, and to treat the inference mechanism as checking the output
of transfer for plausibility (possibly with reference to the domain).
For example, instead of the condition given above on the MRS form,
there could be semantic well-formedness conditions which were applied to
the English logical form, which would exclude the translation of
{\it Termin} as {\it date}.
In the long run this technique might be preferable (though more expensive if
implemented naively) because it is not linked to particular transfer
rules.  However the task still should be seen as the rather directed
one of distinguishing between options for translation, because this
makes it constrained enough to implement.  It is not plausible that a
fully general model of plausibility of a logical form could be used, even
for the restricted domain assumed for Verbmobil.  This alternative is
not attractive with a straightforward version of the lexicalist
approach, because the full LF for the target is not produced prior to
generation.  



\subsection{Inference for alternative expression}

In contrast, consider the following example:
\begin{ex}
\begin{subex}
K\"{o}nnen Sie noch einen anderen Terminvorschlag machen?
\end{subex}
\begin{subex}
Could you make another suggestion for a date?
\end{subex}
\begin{subex}
Could you suggest another date?
\end{subex}
\end{ex}
Here \exnum{0}{b} is more literal, but \exnum{0}{c} might be preferred:
\exnum{0}{b}
is not wrong, but it is rather lumpy.  But if it is
desirable to generate \exnum{0}{c} instead of \exnum{0}{b} some inference is
required to achieve this.

Note that it is plausible to assume that the LF corresponding to
\exnum{0}{b} will actually be
generated by the transfer mechanism unless it is somehow blocked.  It
is necessary to have a translation of {\it Terminvorschlag}
independently of {\it machen} because the sentence could be, for
example:
\begin{ex}
Diesen Terminvorschlag haben wir doch schon letzte Woche diskutiert.\\
We discussed that proposal for the date last week.
\end{ex}
Therefore there are potentially
two ways of generating  \exnum{0}{c}: the LF corresponding
to it could either be constructed by transfer from the German source, or
the generation mechanism could produce it from the LF corresponding to
\exnum{0}{b}.  

In the absence of a generator with a strategic component, the only
option is for the transfer component to produce both possible LFs, and
to impose conditions which will select between them. 
It is difficult to see how a
strategic generation component could be integrated with a
lexicalist approach to transfer. The same problem arises
as that mentioned above with respect to
checking the target LF for semantic plausibility: 
the full LF is only generated by the lexicalist mechanism as part
of the operation of generating the target sentence --- no LF
is output by transfer component.  Thus strategic
generation does not combine well with the lexicalist approach.

This particular example illustrates some potential advantages of
allowing manipulation of the target LF (and therefore rejecting the
lexicalist approach).  As we mentioned in the introduction, the aim is
to produce an architecture where transfer deals with differences
between languages and not with monolingual issues.  It is quite plausible
that a generator for a monolingual system such as a natural language interface
(NLI) could be given the LF
appropriate for \exnum{0}{b}, and that the same issue of possible conversion
to \exnum{0}{c} would arise.  In contrast, it is not likely that
an NLI would produce an LF which contained a predicate which could
either be realised as {\it date} or {\it meeting} --- this is specific
to the translation problem.  Thus there seems to be some motivation for
treating the output of \exnum{0}{c} rather than 
\exnum{0}{b} as a generation rather than a transfer problem.
Another, more theoretical way of looking at this, is that preference
for \exnum{0}{c} over 
\exnum{0}{b} could be seen as exemplifying the Gricean Maxim of
Manner, which is a monolingual principle.  In contrast, preference for
\exnum{+1}{b} as opposed to \exnum{+1}{c} as a translation of
\exnum{+1}{a} is straightforwardly to do with the
content of the utterance.
\begin{ex}
\begin{subex}
Das ist schlecht - da habe ich um 14.30 einen Termin beim
Zahnarzt.
\end{subex}
\begin{subex}
That's bad - I have a dentist's appointment at 14:30.
\end{subex}
\begin{subex}
That's bad - I have a date at the dentist at 14:30.
\end{subex}
\end{ex}

\subsection{Control}
\label{control}

Explicit conditions stated within transfer rules are appropriate
techniques for controlling the application of transfer rules
in many circumstances.  If the job of a semantic
transfer rule is to ensure that denotations are (approximately)
preserved, then the conditions will be restrictions on meaning
of the source or target language predicates, but these may have to
be stated in a variety of ways, since it is impossible to get at
the intended meaning directly. The use of the domain terms mentioned
in \S\ref{mds} is a relatively direct way to get at intended meaning,
which should make it possible to resolve transfer ambiguities in some cases
which analysis of the sentence without reference to the domain
cannot resolve.
In some circumstances
this might be quite difficult --- {\it Termin} as a filler can be
translated as
{\it meeting} or {\it appointment}.
This is a case where
there is a mismatch in meaning: {\it Termin} in this use intersects in
meaning with both {\it meeting} and {\it appointment}.
But the distinction
between {\it appointment} and {\it meeting} 
depends on social factors, such as the relative status of the
participants.  
Working out which was intended might involve an arbitrary amount of
reasoning on real world knowledge, and a better strategy might be to
use a probabilistic method, based on word cooccurrences.

In \S\ref{geben}, when discussing the example of the translation of
{\it geben},
we briefly mentioned the idea of interpreting 
transfer rules as defeasible implications.  In this case, it would be
natural to assume that a more specific transfer rule was applied in
preference to a more general one.  This idea was tried in ELU (Estival
\etal\/,1990).  If condition statements are explicit, the specificity
would presumably be calculated with reference to the conditions, rather
than to the rule as a whole.  It would probably be more appropriate
to formalise this within a logic which is credulous, rather than
skeptical: that is, if two rules both have their antecedents verified
and are not in a mutual specificity relationship, then both rules
should apply.

However, there are cases where it is not appropriate to put explicit
conditions on transfer rules.  For example, the translation of {\it schlecht}
as {\it badly} is unavailable when translating \exnum{+1}{a}
not because there is anything wrong with the presumed meaning of
\exnum{+1}{b}
but because there is a monolingual restriction in English that {\it
suit}, along with other `positive' psychological predicates, cannot
take an adverbial denoting a negative attitude.  
\begin{ex}
\begin{subex}
Das pa\ss t schlecht bei mir.
\end{subex}
\begin{subex}
* That suits me badly.
\end{subex}
\begin{subex}
That doesn't suit me at all.
\end{subex}
\end{ex}
The transfer component
has to provide an alternative, but it is a monolingual condition that
blocks the usual translation.  \exnum{0}{b}
is not (fully) acceptable in any context.  Contrast this with {\it
geben} vs {\it give}: there is nothing {\em linguistically} wrong with
saying {\it you gave me that book}, but it means something different
from the German sentence {\it du gebst mir das Buch}, in some contexts.
Similarly, as we have mentioned before, it is not the responsibility of
a semantic transfer component to deal with all the idiosyncracies of
English preposition selection.  In these cases, it is better to rely on
the generator/target grammar filtering the proposals made by transfer.
This is a very limited step toward the translation by negotiation
proposal, but unless there is a (relatively) clear demarcation of this
sort, it will not be possible to have any sort of principled approach
to transfer.


In principle, therefore, syntactic conditions on semantic transfer
should be unnecessary.  If there is a difference in meaning between two
syntactically distinct usages of a word, this should be reflected in
the semantics.  If there is not, the distinction should be irrelevant
for semantic transfer.  Syntactic conditions indicate that the
monolingual semantic analysis is wrong, or insufficient.  As a
temporary expedient, a semantic transfer component may have to allow
syntactic conditions, but this indicates a flaw in the monolingual
analysis.



\section{Conclusion}

Part of the aim of this paper has been to try and present a view of
semantic transfer as a subject in its own right.  We have
argued that, for the foreseeable future, semantic transfer will be the
best approach to MT for systems such as Verbmobil, because of the limitations
on practical discourse processing and inference.  
We have suggested that some of the problems often assigned to transfer are
actually problems for the monolingual target grammar and generation component.
The hope is that, if we can get the interplay between transfer and
monolingual components right, transfer rules will be less arbitrary,
and it will be easier to write and maintain them.

The three approaches described in detail
in this paper have a considerable amount in common, with respect to
reliance on constraints/unification in a typed framework, specification of
equivalence at a semantic level, modularity,
(relative) independence of application
of transfer rules, ability to cope with
partial interlingua, and (some degree of) lexicalist orientation.
All these features have significant utility.

The MDS transfer component
works efficiently on a range of Verbmobil dialogues,
and makes good use of the constructs developed for semantic construction.
It has certain inelegancies, some of which are due to the restrictions
put on its design by the other components in the 
Verbmobil demonstrator system.
In particular, this approach would probably work better with a
`flatter' semantic representation, such as MRS.
It is clear that some hacks
were necessary to get it to work for the examples, and this
limits confidence in how well it would scale up without modification.  
Minimally, it appears that
some redesign is needed to cope with phrasal translations.
We suggest
that it would be useful in the long term to attempt to separate
the declarative statement of transfer equivalences from the control of their
application.  

Lexicalist machine translation is an attractive approach, which has a 
number of advantages and which has been very influential in
recent work on MT.   
An efficient generation algorithm for Shake-and-Bake
has now been developed (Poznanski \etal\/\ 1995), which removes one of
the obstacles to its adoption.  However
the approach is unable to cover some phenomena without extension and
will not work (in its current form) with grammars which are not 
completely lexicalist.
For long term development, its inability to interact with full flexibility
with an inference and strategic generation
component is a serious disadvantage.  

As we have described here, MRS is primarily an approach to semantic
representation rather than to transfer.  We think that adopting it (or
another approach to flat underspecified semantic representation) will
considerably improve the prospects for writing maintainable and
extensible transfer rulesets.  The specific approach to transfer
suggested in the context of the work on MRS was an attempt to take the
ideas in Shake-and-Bake and to apply them to pure semantic transfer.
From a theoretical viewpoint, it appears that this would go some way
towards correcting the deficiencies noted in Shake-and-Bake, while
maintaining many of the advantages of that method.  For the MRS
approach to transfer to be realistic however, it would have to adopt
some approach to specification of conditions on transfer rules.  It is
also necessary to determine how well MRS itself works with generation
systems and whether the aim of distinguishing between the declarative
statement of transfer equivalences and control principles can be made
to work in practice.



\section*{References}

% first argument - i.e. #1 is always author
% #2 is title
% last is date
% agreed author format is, for example
% Alshawi, H., D. Carter, M. Rayner and B. Gamb\"ack
% 
% the only formatting commands that should appear in fields
% are those to mark idiosyncratic information like accents
\newcommand{\book}[4]{\item #1 (#4) {\it #2,} #3.}
% #3 is publisher
\newcommand{\unpub}[4]{\item #1 (#4) `#2', #3.}
% for tech reports etc
% #3 is information about institution etc
\newcommand{\bookart}[7]{\item #1 (#7) `#2' in #5 (ed.), {\it #4,} #6, pp.~#3.}
% published book article (old \barticle)
% #3 is page numbers 
% #4 book title
% #5 editors
% #6 publishers
\newcommand{\bookartnoed}[6]{\item #1 (#6) `#2' in {\it #4,} #5, pp.~#3.}
% book article where editors are unknown
% #3 is page numbers 
% #4 book title
% #5 publishers
\newcommand{\bookartnopp}[6]{\item #1 (#6) `#2' in #4 (ed.), {\it
#3,} #5.}
% book article where page numbers are unknown
% #3 book title
% #4 editors
% #5 publishers
\newcommand{\forthart}[6]{\item #1 (in press) `#2' in #4 (ed.), {\it #3,} #5.}
% article in forthcoming book (old \farticle}
% #3 book title
% #4 editors
% #5 publishers
\newcommand{\journart}[6]{\item #1 (#6) `#2', {\it #3,} {\bf #4},
#5.}
% article in journal (old \jarticle)
% #3 name of journal
% #4 volume
% #5 pages
\newcommand{\forthjournart}[4]{\item #1 (in press) `#2', {\it #3}.}
% #3 name of journal
\newcommand{\journartnopp}[5]{\item #1 (#5) `#2', {\it #3,} 
{\bf #4}.}
% article in journal no page numbers
% #3 name of journal
% #4 volume
\newcommand{\procart}[6]{\item #1 (#6) `#2', {\it Proceedings of the
#3,} #4, pp.~#5.}
% article in proceedings (old \particle)
% #3 name of conference
% #4 venue
% #5 pages
\newcommand{\pubprocart}[7]{\item #1 (#7) `#2', {\it Proceedings of the
#3,} #4, ,#5, pp.~#6.}
% #3 name of conference
% #4 venue
% #5 publishers
% #6 pages
\newcommand{\procartnopp}[5]{\item #1 (#5) `#2', {\it Proceedings of the
#3,} #4.}
% article in proceedings without known page numbers
% #3 name of conference
% #4 venue
\newcommand{\pubprocartnopp}[6]{\item #1 (#6) `#2', {\it Proceedings of the
#3,} #4, #5.}
% article in proceedings without known page numbers
% #3 name of conference
% #4 venue
% #5 publishers
\newcommand{\procartnoppnoplace}[4]{\item #1 (#4) `#2', {\it Proceedings of the
#3}.}
% article in proceedings without known page numbers without place
% where meeting happened
% #3 name of conference
\newcommand{\procartnoplace}[5]{\item #1 (#5) `#2', {\it Proceedings of the
#3,} pp.~#4.}
% article in proceedings where the venue is not known
% #3 name of conference
% #4 pages
%\newcommand{\acqwp}{ESPRIT BRA-3030 ACQUILEX WP NO.}
%\newcommand{\aunpub}[5]{\item #1 (#4) {\it #2.}\\ \acqwp #3}

{\small
\begin{list}{}
   {\leftmargin 1.6em
    \itemindent -\leftmargin
    \itemsep 0pt plus 1pt
    \parsep 0pt plus 1pt}

\procart{Alshawi, H., D. Carter, M. Rayner and B. Gamb\"ack}
    {Translation by quasi logical form transfer}
    {29th Annual Meeting of the Association for Computational
Linguistics (ACL-91)}
    {Berkeley, California}
    {161--168}
    {1991}


\book{Apresjan, Ju D.}
    {Regular Polysemy}
    {Mouton, The Hague, The Netherlands}
    {1973}

\journart{Arnold, D. and L.~Sadler}
    {The theoretical basis of MiMo}
    {Machine Translation}
    {5 (3)}
    {195--222}
    {1990}

\unpub{Beaven, J.~L.}
    {Lexicalist unification-based machine translation}
    {PhD thesis, University of Edinburgh}
    {1992}

\journart{Brown, P. F., J.~Cocke, S.A.~DellaPietra, V.J.~DellaPietra,
F.~Jelinek, J.D.~Lafferty, R.L.~Mercer and P.S.~Roosin}
{A statistical approach to Machine Translation}
        {Computational Linguistics}
        {16(2)}
        {79--85}
        {1990}

\unpub{Butt, M.}
{Transfer I: Tense and Aspect}
{Verbmobil Report Nr.55, Eberhard-Karls-Universit\"{a}t T\"{u}bingen}
{1995}


\unpub{Copestake, A.}
     {The Compleat LKB}
     {Technical report 316,
University of Cambridge Computer Laboratory}
     {1993}

\procartnopp{Copestake, A. and A.~Sanfilippo}
     {Multilingual lexical representation}
     {AAAI Spring Symposium on Building lexicons for machine translation}
     {Stanford, CA}
     {1993}

\forthjournart{Copestake, A., E.J.~Briscoe, 
P.~Vossen,  A.~Ageno, I.~Castellon, 
F.~Ribas, G.~Rigau, H.~Rodriguez, A.~Samiotou}
{Acquisition of lexical translation relations from MRDs}
{Machine Translation}
{1994}

\journart{Copestake, A. and E.J. Briscoe}
{Semi-productive polysemy and sense extension}
{Journal of Semantics}
{12:1}
{15--67}
{1995}

\procartnopp{Copestake, A., D. Flickinger, R.~Malouf, S.~Riehemann, I.~Sag}
{Translation using Minimal Recursion Semantics}
{The Sixth International Conference on Theoretical and
Methodological Issues in Machine Translation (TMI95)}
{Leuven, Belgium}
{1995}

\unpub{Davis, A.}
    {Linking and the hierarchical lexicon}
    {Doctoral dissertation, Stanford University}
    {forthcoming}

\journart{Dorr, B. J.}
{Machine Translation divergences: a formal description and proposed
solution}
    {Computational Linguistics}
    {20(4)}
    {597--633}
    {1994}

\journart{Emele, M., U.~Heid, S.~Momma and R.~Zajac}
    {Interactions between linguistic constraints: procedural vs.
declarative approaches}
    {Machine Translation}
    {7 (1--2)}
    {61--98}
    {1992}

\procart{Estival, D., A.~Ballim, G.~Russell and S.~Warwick}
    {A syntax and semantics for feature structure transfer}
    {3rd International Conference on theoretical and methodological
issues in MT of NLs (TMI-90)}
    {Austin,Texas}
    {131--143}
    {1990}


\unpub{Frank, A. and U. Reyle}
    {Principle based semantics for HPSG}
    {{\rm to appear in:} Arbeitspapiere des Sonderforschungsbereichs
340, University of Stuttgart}
    {1994}

\book{Hutchins, W.J. and H.L. Somers}
     {An introduction to Machine Translation}
     {Academic Press, London}
     {1992}

% Kamp and Reyle 1993

\book{Kamp, H. and U.~Reyle}
    {From Discourse to Logic}
    {Kluwer Academic Publishers, Dordrecht, Germany}
    {1993}

\procart{Kaplan, R., K.~Netter, J.~Wedekind and A.~Zaenen}
	{Translation by Structural Correspondences}
	{4th Conference of the European Chapter of
	the Association for Computational Linguistics (EACL-89)}
	{Manchester, UK}
	{272--281}
	{1989}

\book{Kay, M., J-M. Gawron and P.~Norvig}
     {Verbmobil: A translation system for face-to-face dialog}
     {CSLI Lecture Notes, No. 33: University of Chicago Press}
     {1994}



\bookart{Landsbergen, J.}
     {Isomorphic grammars and their use in the ROSETTA translation
system}
     {351--372}
     {Machine Translation Today: The State of the Art}
     {M. King}
     {Edinburgh University Press}
     {1987}

\journart{Nirenburg, S. and V. Raskin}
        {The Subworld Concept Lexicon and the Lexicon Management
System}
        {Computational Linguistics}
        {13(3-4)}
        {276--289}
        {1988}

\book{Pollard, C. and I. Sag}
    {Head-driven phrase structure grammar}
    {Chicago University Press, Chicago}
    {1994}

\procartnopp{Poznanski, V., J.L.~Beaven and P.~Whitelock}
     {An efficient generation algorithm for lexicalist MT}
     {33rd Annual Meeting of the Association for Computational
Linguistics (ACL-95)}
     {Cambridge, Mass.}
     {1995}

\unpub{Sag, I.}
    {English relative clause constructions}
    {ms.~Stanford University}
    {1995}

\procartnopp{Sanfilippo, A., E. J. Briscoe, A. Copestake,
M. A. Marti and A. Alonge}
{Translation equivalence and lexicalization in the ACQUILEX LKB}
{4th International Conference on Theoretical and 
Methodological Issues in Machine Translation (TMI-92)}
{Montreal, Canada}
{1992}

\journart{Shieber, S.M.}
    {The problem of logical form equivalence}
    {Computational Linguistics}
    {19 (1)}
    {179--190}
    {1993}

\bookart{Slocum, J., W.S. Bennet, J. Bear, M. Morgan and R. Root}
     {METAL: The LRC Machine Translation system}
     {319--350}
     {Machine Translation today: the state of the art}
     {S.~Michaelson and Y.~Wilks}
     {Edinburgh University Press}
     {1987}

\unpub{Trujillo, I.A.}
     {Lexicalist Machine Translation of Spatial Prepositions}
     {PhD dissertation, University of Cambridge}
     {1995}

\procartnopp{Whitelock, P.}
     {Shake-and-bake translation}
     {14th International Conference on Computational Linguistics (COLING-92)}
     {Nantes, France}
     {1992}

\procart{Zajac, R.}
    {A transfer model using a typed feature structure rewriting system
with inheritance}
    {27th Annual Meeting of the Association for Computational
Linguistics (ACL-89)}
    {Vancouver, BC}
    {1--7}
    {1989}

\end{list}}
\end{document}
% Not fixed

