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\begin{document}
\noindent In Proceedings of the Sixth International Conference on
Theoretical and Methodological Issues in Machine Translation (TMI95),
Leuven, Belgium, 1995\\[0.3in]

\begin{center}
{\LARGE\bf Translation using Minimal Recursion Semantics}\\[0.1in]
{\large\bf Ann Copestake}\\
University of Stuttgart\\
{\large\bf Dan Flickinger, Rob Malouf, Susanne Riehemann, Ivan Sag}\\
CSLI, Stanford University\\
Stanford, CA 94305, USA\\
{\tt \{aac,dan,malouf,sr,sag\}@csli.stanford.edu}
\end{center}


\section*{Abstract}

We describe minimal recursion semantics (MRS), a framework for
semantics within HPSG, which considerably simplifies transfer and
generation.  We discuss why, in general, a semantic representation with
minimal structure is desirable for transfer and illustrate how a
descriptively adequate representation with a non-recursive structure
may be achieved.  The paper illustrates the application of MRS to
transfer with a series of examples and compares the approach to others
which have been previously adopted within unification based frameworks.
Our account involves the use of both language-specific and
interlingual predicates or relations and we illustrate how 
this may be exploited to allow MRS to be
used to investigate different lexical semantic approaches.

\section{Semantic representation and transfer}
\label{intro}

In this paper we describe a semantic representation for HPSG known as
minimal recursion semantics (MRS), which is 
being utilized in the English grammar being developed for the 
Verbmobil project.  Verbmobil is a spoken-dialogue machine
translation system, which is designed to take German or Japanese
input relevant to a limited domain and to produce English output. 
Our aim is to
develop a minimally structured but
descriptively adequate representation, which allows for various types
of underspecification and facilitates generation and the specification
of semantic transfer equivalences.
MRS is not, in itself, a full-fledged semantic theory.  It can perhaps
be best thought of as a meta-level language for describing semantic
structures within HPSG.  
Because MRS supports underspecification, 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.  
The advantages of allowing
various types of semantic underspecification for translation purposes are
well-known (see e.g.~Alshawi et al (1991), Kay et al (1994)), so here
we concentrate on the advantages of flatness or minimal structure
in a semantic representation language.

The problem of ensuring that a grammar can
generate from a particular semantic representation
is well-known: it has 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).  

There are a variety of possible solutions to this general problem within MT.  
Attempts have been made to write (or at least tune) the target language
grammar so that it accepts the output from transfer.  Obviously this
approach is inherently non-modular and unidirectional.
Landsbergen (1987) proposed that the source and target languages be 
made isomorphic, with lexical entries and grammar rules put into correspondence
in order to guarantee that a derivation in the source language would
correspond to one in the target language.  This is a principled approach, but
requires that task-specific source and target grammars be developed 
in parallel.  As Kay et al (1994) point out, it is quite difficult enough to
develop monolingual grammars 
without imposing the requirement that they be isomorphic to a grammar of
another language.
This leads to unnatural analyses and necessarily
means that the grammars be developed together, specifically for MT purposes.
Furthermore, this approach is really only suitable for single language pairs
and cannot be used for a project such as
Verbmobil, where the requirement is to translate from both German and Japanese
into English.

An alternative, which was developed in response to the problems with
the parallel grammar approach,
is Shake-and-Bake translation
(Whitelock 1992, see also Beaven, 1992).
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.  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. 
Poznanski et al (1995) 
show that polynomial complexity in Shake-and-Bake generation can
be achieved by including extra information from the target language
grammar that constrains the
possibilities tried by the generator.  Although this 
potentially dilutes the
original ideal of independent grammars, it still
has advantages over alternative approaches since it could be made clear
that this is control information.  Even so, Shake-and-Bake 
(and related systems such as those described by 
Sanfilippo et~al~(1992) and Trujillo (1995)) have other 
disadvantages inherent to strongly lexicalist approaches.
Representing phrasal equivalences is cumbersome, 
since they have to be 
stated in terms of sets of monolingual entries.  
The requirement that grammars be absolutely lexicalist 
restricts the frameworks for which it is appropriate and is even in 
conflict with recent proposals
within HPSG (Sag 1995).
As we will illustrate in \S\ref{schlecht}, some translation equivalences
cannot be encoded simply by relating indices in a conventional semantic 
framework, and therefore cannot be treated without expansion 
and complication of the original approach.
Finally, 
translations tend to seem very literal because
of the reliance on word to word equivalence and it is difficult to see
how a strongly
lexicalist approach could accommodate more flexible translations.

It is more usual to insist
that transfer produces a LF which is syntactically as well as semantically 
equivalent to a LF accepted by the generator.  
Several systems which pursue this approach allow the transfer component
to specify both syntax and semantics (e.g.~Estival et al,
1990, Arnold and Sadler, 1990) but because we are interested in
minimizing the extent to which the writers of the transfer component
need knowledge of the target language grammar, we assume that transfer
only specifies the semantics. 
A semantic transfer system can be built
without developing special purpose grammars, 
at least for fairly small systems, 
as the SRI 
spoken language translation BCI system 
(Alshawi, 1992; Alshawi et al 1991)
demonstrates.  However transfer is much more complex than with
Shake-and-Bake and
the transfer rules must be written by someone with a detailed 
knowledge of the target language grammar,
since under most proposals for semantic 
representation the syntactic form of the LF is partially dependent on 
constituent structure.  The BCI system uses semantic transfer at the
level of quasi-logical form (QLF): pronouns and scope ambiguities are
not resolved, for example.
Alshawi et al motivate this decision to use a relatively `surfacy'
representation in detail --- the essential point is that ambiguity
resolution can be computationally expensive and/or domain dependent and
that it should therefore not be carried out unless necessary for
translation.  
However, from our current viewpoint, the QLF representation still
retains some unnecessary structure and some examples
of transfer have proved difficult to implement in the BCI system.

The approach proposed here can be regarded as taking some elements from
a BCI-like approach to semantic transfer and some from Shake-and-Bake
(and the variant on that approach described by Trujillo (1995)).
Its feasibility rests on two things.  The first is 
the observation that translation is not about strict preservation of
denotation. This raises many problems
but it does indicate that we do not have to
expect that the generator tests all logically equivalent forms of the 
expression generated by the transfer component, or that the transfer component
produces all possible logically equivalent forms acceptable to the
target grammar.  
The second is that, although
the theoretical decidability of logical equivalence is independent
of the syntax of the representation language, in engineering terms the
ease of constructing appropriate forms is highly dependent on the
syntax.  To return to the example \exnum{0}{}
above, the strictly binary nature of
$\wedge$ leads to a spurious ambiguity in representation, because the
bracketing is irrelevant to the truth conditions.  But the generator
can only determine this by examining the logical properties of
$\wedge$.  An alternative representation could have been adopted
which does not introduce spurious semantic ambiguity by redundant bracketing.
For example, instead of \exnum{0}{a} (repeated as 
\exnum{+1}{a}) we could have written \exnum{+1}{b} where $\wedge$
takes a set of arguments.  
To avoid the computational expense
of deciding whether two sets have the same members we could agree to give 
this the canonical form shown in \exnum{+1}{c} where a conventional 
order is imposed on the propositions.
\begin{ex} 
\begin{subex}
$\lambda x [ \mbox{fierce}(x) \wedge (\mbox{black}(x) 
\wedge \mbox{cat}(x))  ]$ 
\end{subex}
\begin{subex}
$\lambda x [ \bigwedge \{\mbox{fierce}(x), \mbox{black}(x), \mbox{cat}(x)\}  ]$ 
\end{subex}
\begin{subex}
$\lambda x [ \bigwedge \{\mbox{black}(x), \mbox{cat}(x), \mbox{fierce}(x) \}  ]$ 
\end{subex}
\end{ex}

Consider the effect this has on transfer from
Spanish to English.  Instead of a series of 
recursively applied rules which either overgenerate
with concomitant lack of efficiency or have to be constructed with 
knowledge of the English grammar of adjectives, we can
have a simple, non recursive procedure which generates \exnum{0}{c}
purely on the basis of the simple transfer equivalences 
$\mbox{black}(x) \leftrightarrow \mbox{negro}(x)$,  
$\mbox{fierce}(x) \leftrightarrow \mbox{feroz}(x)$ and
$\mbox{cat}(x) \leftrightarrow \mbox{gato}(x)$.  
More importantly, it becomes much easier
to state those equivalences for which there is no one to one mapping between
lexically realized predicates.  For example, for the equivalence 
English:{\it young bull} $\approx$ Spanish:{\it novillo}, we can simply state
$\{ \mbox{bull}(x), \mbox{young}(x)\} \leftrightarrow \{ \mbox{novillo}(x) \}$, 
without having to worry
about the possibility of interpolated material (e.g.~{\it young black bull}).

In general, the greater the structural complexity of the
representation, the worse the problem of constructing a LF acceptable
to the target grammar becomes.  
A separate, though not unrelated, issue is that the semantic
representation chosen should allow for underspecification of the
sorts of information which are not straightforwardly resolvable when
analyzing natural languages, where the ambiguity can be naturally preserved
on translation.\footnote{For English/German,
this is often true for
quantifier scope and PP attachment ambiguities, but does not, in
general, apply to lexical ambiguity, except for some semi-regular cases
of polysemy which hold cross-linguistically, 
such as the abstract representation/physical object sense of {\it
newspaper}.}
The problem then is to define a semantic representation which is 
logically adequate but not
unnecessarily structured, which reflects the properties that 
translation should aim to preserve and which allows for
underspecification.  A final essential criterion is that the representation
should be capable of supporting inference.  

The proposal for the
translation mechanism made here can be classified as semantic 
transfer.\footnote{This is a matter of terminology, since 
there is no sharp dividing line between semantic transfer and 
those interlingual approaches which do not insist on the identity of
source and target language representations.  
We will assume that some predicates
are interlingual and some language-specific in what follows.}  It relies on
the transfer component constructing a representation which can be 
accepted by the target grammar, with some limited
contribution from the generator.
The transfer component will sometimes output multiple forms,
some of which may be unacceptable to the target grammar
(a technique referred to as filtering in the BCI system).  
There will frequently be multiple possible inputs to the generator, but these
will be ordered by a control mechanism which is distinct from
the declarative transfer equivalence specification.  
Further details are given in the rest of this paper, after the
description of MRS.

\section{An introduction to MRS}
  
We begin by discussing some general issues in achieving a minimally
structured semantic representation.
We will assume a neo-Davidsonian style of representation
with explicit event variables.  This immediately leads to
a rather flat representation style: contrast \exnum{+1}{b,c} with
\exnum{+1}{d} (these examples and others in this introductory section
are for expository purposes, not a serious proposal for
the representation of temporal adverbials etc):
\begin{ex}
\begin{subex}
On Monday Kim ran in Foothills Park.
\end{subex}
\begin{subex}
on(in(run(Kim), Foothills\_Park), Monday)
\end{subex}
\begin{subex}
in(on(run(Kim), Monday), Foothills\_Park)
\end{subex}
\begin{subex}
on($e$, Monday) $\wedge$ run($e$, Kim) $\wedge$ in($e$, Foothills\_Park)
\end{subex}
\end{ex}
The structure in \exnum{0}{d}
is more appropriate because it does not introduce the
spurious scoping distinction.

The most important use of recursive structures in a semantic
representation is to allow for scope.
For example, 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}
$\mbox{every}(x, \mbox{man}(x) \wedge \mbox{tall}(x), \mbox{old}(x))$ 
\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}
$\bigwedge\{\mbox{every}(x), \mbox{man}(x), \mbox{old}(x), \mbox{tall}(x)\}$ 
\end{ex}
\begin{ex}
Every old man is tall.
\end{ex}
We therefore require a flat representation 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.

We can achieve this effect 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, we simply tagged everything which would
have been inside a set of braces in a conventional formula 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, we need to be able to represent the relative scope of
pairs of quantifiers (Frank and Reyle, 1994).  This can be accomodated
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 sort {\typef mrs-struct}, 
with appropriate features {\sc handel} 
and {\sc liszt}, which take values of sort {\typef handle} 
and {\typef list} respectively.  
We use the feature name
{\sc liszt} 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 and {\sc liszt} is an
appropriate name for an extraordinary object which composes the
semantics.  In keeping with this theme,
since handle sounds rather plebeian, we use
{\sc handel} for the other main feature of the compositional semantics.
We adopt the normal convention of writing feature names in (small)
capitals and sorts/types 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}.
Verb rels have a feature {\sc event} which takes 
an event variable.  
They
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}.
We will introduce some other specialized subsorts of rel later in this 
document.  
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 we have not assigned a scope: 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 et al (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).
The MRS representation of \exnum{+1}{} is shown in Figure~\ref{suit}: we
will use this example in the discussion of translation in the next section. 
\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, we 
unify the handels 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. 
Becaause 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.  The nominal senses of {\it suit} will
have different rels, of course.
The event e8 is actually a state, but we will not draw any distinctions
between eventuality types for the purposes of this paper.  We also
omit any representation of tense here, since MRS is
compatible with a variety of approaches.  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. Similarly we assume that tense rels take event arguments.

\section{Translation}
\label{transexs}

Translation between MRS representations basically depends on setting up
the correct coindexations between the arguments of the
rels.\footnote{For simplicity, we show coindexation between 
the complete indices here, but the true 
representation is slightly more complex, since in HPSG indices contain 
information which will not be shared between
languages, such as gender values, for example.}
A trivial example is shown
in the Figure~\ref{easytrans}, where all the mappings between relations
are one-to-one.  In this section, we will use the convention that
German MRS expressions are shown on the left of the transfer equivalence
and English on the right.  The equivalences are all symmetric and
bidirectional, however.
By making transfer equivalences themselves be 
FSs, we can use reentrancies between the 
two halves of the equivalences
to indicate argument identification and
specify generalizations about classes of transfer equivalence using
sorts.
Note that the output MRS is underspecified, in that the top-level
{\sc handel} and {\sc index} values are not instantiated, but 
constraints on the English grammar would mean that
h1 and e4 are the only possible values.
\begin{figure}
{\small
\begin{tabular}{ll}
Input: & {\it Kim singt}\\[0.2in]

Input MRS: & 
\begin{avm}
  \[ handel & \@{h1}\\
    index & \@{e4}\\
    \avmspan{liszt \;
    \<  \osort{named\_rel}
       {\[ handel & \@{h1}\\
           name & Kim\\
           named & \@{x2}\]},
        \sort{singen\_rel}
       {\[ handel & \@{h1}\\
           event & \@{e4}\\
           act & \@{x2} \]} \>} \] 
\end{avm}\\[0.2in]

Transfer equivalences: &
\begin{avm}
\sort{singen\_rel}
{\[  handel & \@{h2}\\
     event & \@{e3}\\
     act & \@{x1} \]}

$\transarrow$

\sort{sing\_rel}
{\[  handel & \@{h2}\\
     event & \@{e3}\\
     act & \@{x1} \]}
\end{avm} \\

&
\begin{avm}
\sort{named\_rel}
{\[  handel & \@{h3}\\
     name & \@{n1}\\
     named & \@{x4} \]}

$\transarrow$

\sort{named\_rel}
{\[  handel & \@{h3}\\
     name & \@{n1}\\
     named & \@{x4} \]}
\end{avm} \\[0.2in]

Instantiated equivalences: &
\begin{avm}
\sort{singen\_rel}
{\[  handel & \@{h1}\\
     event & \@{e4}\\
     act & \@{x2} \]}

$\transarrow$

\sort{sing\_rel}
{\[  handel & \@{h1}\\
     event & \@{e4}\\
     act & \@{x2} \]}
\end{avm} \\

&
\begin{avm}
\sort{named\_rel}
{\[  handel & \@{h1}\\
     name & \@{n1} Kim\\
     named & \@{x2} \]}

$\transarrow$

\sort{named\_rel}
{\[  handel & \@{h1}\\
     name & \@{n1}\\
     named & \@{x2} \]}
\end{avm}\\[0.2in]


Output MRS:  &
\begin{avm}
  \[ handel & \\
    index & \\
    \avmspan{liszt \;
    \<  \sort{named\_rel}
       {\[ handel & \@{h1}\\
           name & Kim\\
           named & \@{x2}\]},
        \sort{sing\_rel}
       {\[ handel & \@{h1}\\
           event & \@{e4}\\
           act & \@{x2} \]} \>} \] 
\end{avm}\\[0.2in]

Output: & {\it Kim sings}
\end{tabular}}
\caption{Transfer using MRS}
\label{easytrans}
\end{figure}

Most 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}
\label{schlecht}

All transfer equivalences consist of relationships between sets of rels but
the vast majority will be lexically motivated.
We can distinguish two subcases:
lexical transfer equivalences, where the liszts 
which are related by the equivalence will each be found in
a lexical sign, and phrasal transfer equivalences, where one or more of the
liszts can only be realized as (part of) a phrasal sign.
For example, the
following are examples of lexical transfer equivalences that could apply in
the translation of \exnum{+1}{a} (which has the MRS representation shown
in Figure~\ref{passen})
to \exnum{0}{} (repeated as
\exnum{+1}{b}):
\begin{ex}
\begin{subex}
Das pa\ss t echt schlecht bei mir
\end{subex}
\begin{subex}
That really doesn't suit me well
\end{subex}
\end{ex}
\begin{center}
{\small
\begin{avm}
\< \sort{passen\_rel}
{\[  theme & \@{i1}\\
     exp & \@{x2} \]} \>

$\transarrow$

\< \sort{suit\_rel}
{\[  act & \@{i1}\\
     und & \@{x2} \]} \>
\end{avm}\\[0.2in]
\begin{avm}
\< \sort{echt\_rel}
{ \[  arg & \@{h1} \]} \>

$\transarrow$

\<\sort{real\_rel}
{\[  arg & \@{h1} \]} \>
\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 {\it tlinks} described by Copestake
and Sanfilippo (1993) and Sanfilippo et al (1992)).
But in certain examples,
event variables may not correspond one-to-one across languages.
We will discuss one case in \S\ref{river}.
\begin{figure}
\begin{center}
{\small
\begin{avm}
  \[ handel & \@{h1}\\
    index & \@{e8}\\
    \avmspan{liszt \;
    \<  \osort{schlecht\_rel}
       {\[ handel & \@{h3}\\
           arg & \@{e8} \]},
       \sort{passen\_rel}
       {\[ handel & \@{h5}\\
           event     & \@{e8}\\
           theme & \@{i7}\\
           exp & \@{x6} \]}, \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}

A simple phrasal transfer equivalence, for {\it
schlecht}$\approx${\it not good} is given below:\footnote{There will also be an
equivalence {\it schlecht}$\approx${\it bad}, of course, but the
equivalence with {\it not good} seems justified by the regularity with
which {\it schlecht} is used in contexts where {\it bad} is an
inappropriate translation.  We discuss how we avoid the translation of
\exnum{-1}{a} as {*\it that really suits me badly} in \S\ref{filtering}.}
\begin{center}
{\small
\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}
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).
Shake-and-Bake as described in Whitelock (1992) and Beaven (1992) could
not represent this example adequately because it does not have transfer
variables and the semantic
representation assumed has nothing equivalent to handels.

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.

\subsection{Generation and control}
\label{filtering}

Since transfer produces a logical form, standard techniques
for generating from a semantic representation can be used with MRS.
Lexical entries can be indexed by rel name for efficient lookup.
If the grammar and transfer were completely lexicalist --- i.e. if the 
rels generated by transfer
corresponded directly to single lexical entries,
then the translation process could actually be 
implemented in a manner very similar to Shake-and-Bake,
since unifying the rels with the semantics of the relevant lexical
entry would give an instantiated lexical sign.
However, because MRS decouples the lexicon and the semantics, more flexible
strategies are possible, while retaining 
the advantages of lexicalism where it is appropriate.
The creation of a complete target logical form prior to
generation of the target string has the advantage over the Shake-and-Bake 
style approach that a strategic generator can check
the logical form and either request an alternative from transfer or
rewrite it itself,
if that appears appropriate. 

For instance, taking the example above, we 
wish to avoid the generation of \exnum{+1}{}:
\begin{ex}
* That really suits me badly
\end{ex}
The semantic representation corresponding to this will be output by the
transfer equivalences, but it is excluded on monolingual grounds, since
verbs which denote a positive attitude 
towards something such as {\it suit}, {\it like} and so on cannot be
modified by negative adverbials (Condoravdi and Sanfilippo, 1990).  
Since for this case the unacceptability can be determined by
examination of the target logical form alone, the filtering could be
done prior to generation proper for efficiency.    

The transfer equivalences given above allow {\it schlecht} to be
translated as {\it not well} in any context where it is modifying an
event.  This is sometimes desirable, but usually should be blocked by
the translation as {\it badly}.  
As a general heuristic, we can specify that translations which produce 
single lexical items are to be preferred over those 
which produce phrases.   
This selection
could reasonably be part of the functionality of the generator, rather than
the transfer component, since it
essentially corresponds to the monolingual principle of
avoiding periphrasis and 
is dependent on the number of lexemes involved, not on the number of
rels.
But, since in general we assume that the transfer
component incorporates heuristics which order translation equivalences,
it might be
more practical to include a heuristic which avoids
equivalences which have phrases on the target side if 
alternative lexical outputs are available.

\subsection{A more complex example}
\label{river}

To facilitate comparison with other approaches to transfer, we now
consider the standard example of the translation of
English verbs of manner of motion (e.g.~~{\it swim}, {\it stagger})
occurring with a locative expression
describing a completed path.
In many other languages, including the Romance languages and Japanese,
this pattern is not possible (Talmy 1985).  Here we will concentrate on
Spanish, 
where the manner of motion is expressed with an adverbial, while the
main verb conveys the path.  For example,
\exnum{+1}{a} is usually translated as
\exnum{+1}{b} (which is more literally translated as \exnum{+1}{c}). 
\begin{ex}
\begin{subex}
Kim swam across the river
\end{subex}
\begin{subex}
Kim cruz\'{o} el r\'{\i}o nadando
\end{subex}
\begin{subex}
Kim crossed the river swimming
\end{subex}
\end{ex}
In order to give a direct comparison between transfer using MRS and
an alternative sign-based approach, we will
use a comparable semantic analysis to that given by Sanfilippo et al
(1992).\footnote{For some other approaches to this mismatch problem see,
for example,
Isabelle et al, 1988; Beaven, 1992; Nirenburg and Levin, 1992; Dorr, 1994.}
In this analysis, the gerundive is translated as expressing a distinct
event, which is related to the main verb event by a predicate {\bf
while}.  For current purposes, the distinction between MRS and
Sanfilippo et al's encoding of a neo-Davidsonian semantics by
use of proto-roles is not significant.  We show a linearized equivalent
of their encoding of the
semantics for {\it nadando}\footnote{We have omitted the classification
of events which Sanfilippo et al adopt --- this is an
important part of the analysis but is not relevant to the comparison
with MRS since
we could implement it in exactly
the same way, by typing event variables.  
Apart from this, we have essentially preserved the analysis,
but we do not reproduce the original FS here, since 
it is multiply nested, due to the use of a binary {\bf and} and over
20 lines of text.}
and the
corresponding MRS representation below:
\[ [ e2 ] [ \mbox{while}(e2, e1) \wedge \mbox{nadar}(e2) \wedge
\mbox{p-agt-cause-move-manner}(e2,x1) \wedge P(e1) ] \]
\begin{center}
{\small
\begin{avm}
  \[ handel & \@{h1}\\
    index & \@{e2}\\
    \avmspan{liszt \;
    \<  \osort{while\_rel}
       {\[ handel & \@{h1}\\
           arg1 & \@{e2}\\
           arg2 & \@{e1}\]},
       \sort{nadar\_rel}
       {\[ handel & \@{h1}\\
           event     & \@{e2}\\
           act & \@{x1} \]},
     \sort{rel}
       {\[ handel & \@{h1}\\
           event & \@{e1} \]}
\>}\]
\end{avm}}
\end{center}
A simplified form of the MRS representations
for \exnum{0}{a} and \exnum{0}{b} are
shown in Figure~\ref{trans}. %  
\begin{figure}
\begin{center}
{\small
\begin{avm}
  \[ handel & \@{h1}\\
     index & \@{e1}\\
    \avmspan{liszt \;
    \< \osort{named\_rel}
       {\[ handel & \@{h1}\\
           name & Kim\\
           named & \@{x1}\]},
       \sort{swim\_rel}
       {\[ handel & \@{h1}\\
           event     & \@{e1}\\
           act & \@{x1} \]}, \avmr}\\ 
     \avmspan{\hspace{0.3in}\;\avml
     \sort{across\_rel}
       {\[ handel & \@{h1}\\
           arg1 & \@{e1}\\
           arg2 & \@{x2}\]},
     \sort{the\_rel}
       {\[ handel & \@{h1}\\
           bv & \@{x2}\\
           restr & \@{h2}\]},
       \sort{river\_rel}
       {\[ handel & \@{h2}\\
           inst & \@{x2} \]}
\>}\]
\end{avm}}
\end{center}
\begin{center}
{\small
\begin{avm}
  \[ handel & \@{h1}\\
    index & \@{e1}\\
    \avmspan{liszt \;
    \< \osort{named\_rel}
       {\[ handel & \@{h1}\\
           name & Kim\\
           named & \@{x1}\]},
        \sort{while\_rel}
       {\[ handel & \@{h1}\\
           arg1 & \@{e2}\\
           arg2 & \@{e1}\]},
       \sort{nadar\_rel}
       {\[ handel & \@{h1}\\
           event     & \@{e2}\\
           act & \@{x1} \]},\avmr}\\ 
     \avmspan{\hspace{0.3in}\;\avml
     \sort{cruzar\_rel}
       {\[ handel & \@{h1}\\
           event & \@{e1} \\
           act & \@{x1}\\
           path & \@{x2}\]},
     \sort{el\_rel}
       {\[ handel & \@{h1}\\
           bv & \@{x2}\\
           restr & \@{h2}\]},
       \sort{rio\_rel}
       {\[ handel & \@{h2}\\
           inst & \@{x2} \]}
\>}\]
\end{avm}}
\end{center}
\caption{Simplified MRS representations for {\it Kim swam across the river}
and {\it Kim cruz\'{o} el r\'{\i}o nadando}}
\label{trans}
\end{figure}%
We assume that strict intransitive 
manner of motion verbs such as {\it swim} are related to entries
subcategorized for a PP expressing a path by lexical rule.  The details
of this are unimportant here, but we assume that in Spanish this rule
cannot apply to manner of motion verbs when the path is bounded.
In contrast to Sanfilippo et al's use of a Shake-and-Bake style
representation of transfer equivalence between sets of lexical signs,
we make use of the transfer equivalences 
between liszts shown below (in this section,
we are using the convention that the English rels are on the left of
the translation equivalence).
\begin{center}
{\small
\begin{avm}
\< 
\sort{swim\_rel}
{\[  event & \@{e1}\\
     act & \@{x1} \]},
\sort{path\_rel}
       {\[ arg1 & \@{e1}\]} \>
$\transarrow$\end{avm}\hfill\mbox{}\nopagebreak\\ 
\nopagebreak
\mbox{}\hfill
\begin{avm}
\<\sort{nadar\_rel}
{\[  event & \@{e2}\\
     act & \@{x1} \]},
\sort{while\_rel}
       {\[ arg1 & \@{e2}\\
           arg2 & \@{e1}\]}, 
\sort{move\_path\_rel}
       {\[ event & \@{e1} \\
           act & \@{x1}\]} \>
\end{avm}\\[0.05in]
\begin{avm}
\< 
     \sort{across\_rel}
       {\[ arg1 & \@{e1}\\
           arg2 & \@{x2}\]},
       \sort{move\_rel}
       {\[ event     & \@{e1}\\
           act & \@{x1} \]}\>
$\transarrow$
\<     \sort{cruzar\_rel}
       {\[ event & \@{e1} \\
           act & \@{x1}\\
           path & \@{x2}\]} \>
\end{avm}
}
\end{center}
In these rules we have made use of the underspecified rels 
{\typef path\_rel}, {\typef move\_path\_rel} and {\typef
move\_rel} which subsume 
{\typef across\_rel}, {\typef cruzar\_rel} and {\typef
swim\_rel} respectively.  These constrain the transfer equivalences in
a general manner, so that the former equivalence will apply to
a range of paths and the latter to a range of manner of movement verbs.
The easiest way to understand these underspecified equivalences is to assume
that they are unified in advance of transfer, to give a fully 
instantiated transfer equivalence, as shown below:
\begin{center}
{\small
\begin{avm}
\<\sort{swim\_rel}
{\[  event & \@{e1}\\
     act & \@{x1} \]},
     \sort{across\_rel}
       {\[ arg1 & \@{e1}\\
           arg2 & \@{x2}\]}\>
$\transarrow$ 
\end{avm}\hfill\mbox{}\\
\mbox{}\hfill
\begin{avm}
\<      \sort{nadar\_rel}
        {\[  event & \@{e2}\\
             act & \@{x1} \]},
\sort{while\_rel}
       {\[ arg1 & \@{e2}\\
           arg2 & \@{e1}\]},
     \sort{cruzar\_rel}
       {\[ event & \@{e1} \\
           act & \@{x1}\\
           path & \@{x2}\]}\>
\end{avm}}
\end{center}
Together with the straightforward equivalences between {\it river} and
{\it rio} etc, this enables  \exnum{0}{a} to be translated as \exnum{0}{b}
(or vice versa).

The transfer equivalence 
for {\it swim} can be generated automatically from the
usual equivalence between 
{\typef swim\_rel} and {\typef nadar\_rel} shown below.
\begin{center}
{\small
\begin{avm}
\<\sort{swim\_rel}
{\[  event & \@{e1}\\
     act & \@{x1} \]}\>
$\transarrow$
\<\sort{nadar\_rel}
{\[  event & \@{e1}\\
     act & \@{x1} \]}\>
\end{avm}}
\end{center}
This is achieved by making use of a metalevel
pattern (shown below) along the lines of the {\it tlink-rules} described by 
Copestake and Sanfilippo (1993), which will
also produce the more complex equivalence from other manner of movement
verbs equivalences such as 
English:{\typef float}$\approx$Spanish:{\typef flotar}.  
Here {\typef manner\_move\_rel} is a sort which
subsumes {\typef swim\_rel}, {\typef float\_rel}, {\typef nadar\_rel}
etc and thus the translation equivalence between 
{\typef swim\_rel} and {\typef nadar\_rel} will instantiate the top
half of the structure, generating the new equivalence.
\begin{center}
{\small
\begin{avm}
\<\sort{manner\_move\_rel}
{\[  event & \@{e1}\\
     act & \@{x1} \]}\>
$\transarrow$
\<\sort{manner\_move\_rel}
{\[  event & \@{e1}\\
     act & \@{x1} \]}\>
\end{avm}\nopagebreak\\[0.05in]
{\large\boldmath $\Downarrow$}\nopagebreak\\[0.05in]
\nopagebreak
\begin{avm}
\<\sort{manner\_move\_rel}
{\[  event & \@{e1}\\
     act & \@{x1} \]},
\sort{path\_rel}
       {\[ arg1 & \@{e1}\]}\>
$\transarrow$
\end{avm}\hfill\mbox{}\nopagebreak\\ 
\nopagebreak
\mbox{}\hfill
\begin{avm}
\<\sort{manner\_move\_rel}
{\[  event & \@{e2}\\
     act & \@{x1} \]},
     \sort{move\_path\_rel}
       {\[ event & \@{e1} \\
           act & \@{x1}\]}
 \sort{while\_rel}
       {\[ arg1 & \@{e2}\\
           arg2 & \@{e1}\]}\>
\end{avm}}
\end{center}
It is essentially this meta-pattern
which encapsulates the generalization about translation.  Like a
tlink-rule, it can be implemented straightforwardly within a
unification based framework if transfer equivalences are themselves FSs.  

The MRS
representation is considerably simpler than that of
Sanfilippo et al, though for reasons of space we do not show their
transfer equivalences here.  Unlike that approach, it does not specify
the grammatical means employed in Spanish to create the adverbial
manner component.
Alternative semantic analyses are possible, however. 
It might be
preferable to introduce an interlingual notion of paths, 
since there
seem to be a limited number of possibilities for lexically encoded
paths, which would allow us to avoid expressing the transfer
equivalence between {\it cruzar} and {\it across}.
Furthermore, it would be
possible to analyze English sentences such as \exnum{0}{a} 
as having two event variables.  This would give a semantics
more similar to that proposed by Talmy (1985). 
Thus we could take
an approach within MRS which was closer to an interlingual
representation (see e.g.~Nirenburg and Levin (1992) and Dorr (1994) for a more
interlingual approach to this example), 
if monolingual criteria seemed to justify the move. 
We will not go into details here
but we believe that our use of MRS makes it
much more straightforward to concentrate on the
purely semantic aspects of representation and transfer
problems and hence to investigate a variety of 
approaches to lexical semantics.

\section{Conclusion}

We have shown here how MRS representations simplify the description of 
transfer equivalences 
and alleviate the problem of ensuring that the grammar
generates from the output of transfer.  The potential cost of doing
this is in efficiency, since the generator cannot take advantage of any
clues about the syntax that may be implicit in a more structured
representation.  However, we believe that control statements should be stated
explicitly, both to avoid complicating the transfer component and
because this will lead to greater efficiency for a
particular generation strategy.  There will
normally be several possible sentences that can be generated from any
one semantic representation output by the transfer component.  
In principle this is desirable to avoid stilted translations:
sentences would be ordered by considering
monolingual information such as preferred collocations and
constructions and by explicitly identifying the discourse effects
that affect word order.  In practice we would expect that
heuristics would be used (such as preferring 
the word order that corresponds most closely with the source language).

As we mentioned in \S\ref{filtering}, the approach to transfer
described here would be similar to Shake-and-Bake,
if there were a one-to-one relationship between rels and lexical items.
But as we have illustrated here, the MRS approach allows a less cumbersome
treatment of some phrasal translation equivalences and facilitates 
investigating representations which are partially interlingual.
We also believe that there will be long-term advantages in avoiding
direct linkage between semantics and lexical items, because it
makes it possible to incorporate more flexible translation and
generation strategies, 
for the cases where a lexical approach gives stilted or unnatural results.
This is currently most relevant for 
domain-specific speech-based MT systems, such as Verbmobil, where it is
feasible to incorporate at least limited inference on a domain model,
and where it is especially important to be able to generate 
natural-sounding output.

As we said in the introduction, MRS is
intended as a metalevel representation which is compatible with 
semantic theories such as DRT.  In this paper, we have concentrated on 
illustrating the advantages of a flat semantics for transfer, but
MRS also allows underspecification of quantifier scope and PP attachment
(by underspecification of arguments) and of lexical ambiguity
(by use of a hierarchy of rel types).


\section*{Acknowledgements}
We are grateful to Martin Kay, Christian Rohrer and Arturo Trujillo for
ideas and comments on this paper.
This work was partially funded by the German Federal Ministry 
for Research and Technology (BMFT) in the framework of the
Verbmobil Project. 
The responsibility for the contents of this study lies with the
authors.


\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 (#6, forthcoming) `#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{\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}

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\procartnopp{Copestake, A. and A.~Sanfilippo}
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\unpub{Copestake, A., D.~Flickinger, R.~Malouf, S.~Riehemann, I.~Sag}
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\unpub{Davis, A.}
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\procartnopp{Isabelle, P., M. Dymetman and E.~Macklovitch}
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\book{Kay, M., J.M. Gawron and P. Norvig}
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\bookart{Landsbergen, J.}
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\bookart{Nirenburg, S. and L. Levin}
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the first SIGLEX Workshop, Berkeley, 1991}
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\book{Pollard, C. and I. Sag}
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\procartnopp{Poznanski, V., J.L.~Beaven and P.~Whitelock}
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Linguistics (ACL-95)}
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\unpub{Sag, I.}
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\procartnopp{Sanfilippo, A., E. J. Briscoe, A. Copestake,
M. A. Marti and A. Alonge}
{Translation equivalence and lexicalization in the ACQUILEX LKB}
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Methodological Issues in Machine Translation (TMI-92)}
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\journart{Shieber, S.M.}
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\bookart{Talmy, L.}
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\end{list}}
\end{document}