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\begin{document}
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\vspace*{1in}
{\large\bf
\begin{tabbing}
ACQUILEX II --- Esprit Project 7315\\[0.2in]
Working paper number: 3\\[0.2in]
Date: January 1993\\[1in]
Title: Multilingual Lexical Representation\\[0.3in]
Authors:   \= Ann Copestake and Antonio Sanfilippo\\[0.1in]
\> Computer Laboratory, University of Cambridge\\[2in]
In: `Building Lexicons for Machine Translation', 
edited by Bonnie Dorr,\\ 
Proceedings of the AAAI Spring Symposium, Stanford, California, 1993
\end{tabbing}}
\newpage
\setcounter{page}{1}
\begin{center}
{\Large\bf Multilingual Lexical Representation}\\
{\large Ann Copestake and Antonio Sanfilippo}
\end{center}

\begin{abstract}
The approach to multilingual lexical representation developed as part of the
ACQUILEX Lexical Knowledge Base (LKB) is discussed with specific reference to
complex translation equivalence. The treatment described provides a lexicalist
account of translation mismatches in terms of translation links which capture
cross-linguistic generalizations across sets of semantically related lexical
items, and can be readily integrated with several transfer-based MT systems.  
\end{abstract}

\section{Introduction}

The ACQUILEX LKB system was designed to allow the representation of
syntactic and semantic information which has been (semi-)automatically
extracted from machine readable dictionaries (MRDs). Large scale monolingual
lexicon fragments have been constructed semi-automatically for four languages
(English, Spanish, Dutch and Italian); descriptions of the monolingual
lexicons and the lexical representation language (LRL) are given in, for
example, Copestake (1992), Sanfilippo and Poznanski (1992) and papers in
Briscoe {\it et al.}~(in press).  Here we describe our use of the LRL to
represent multilingual information in the form of links between monolingual
lexical entries, and discuss how this representation is usable by MT
systems.  We regard MT as only one possible application; ultimately the
multilingual LKB should be able to support the needs of linguists,
lexicographers and others who need access to substantial quantities of
cross-linguistic information.

Recently, there has been considerable interest in using unification
based formalisms to model transfer approaches to MT, aiming for
declarativeness and bidirectionality, but allowing sufficient
expressiveness to deal with complex classes of translation equivalence
(e.g. Kaplan {\it et al.}, 1989; Zajac, 1989; Estival {\it et al.},
1990; Alshawi {\it et al.}, 1991). 
We have attempted to maintain these advantages,
but to abstract away from the aspects of these systems which are
specific to particular MT techniques.
This makes it possible to maximize the functionality of the system with
respect to the expression of linguistic and lexicographic
generalisations and facilitates the construction of a multilingual
lexicon which would support a variety of approaches to MT, although
it is naturally most appropriate to the more lexicalist frameworks.

Like parsing and generation, MT can be regarded very generally as a process of
constraint solving.  The target language (TL) sentence generated is 
constrained by the monolingual grammar and lexicon and also by
constraints derived from the parse of the source language (SL)
sentence.  
Different
approaches to MT can be characterised according to what sort of
cross-linguistic constraints are imposed, in addition to 
the monolingual constraints.  
The archetypal use of an interlingua
corresponds to the situation where the structure produced by the parse
of the SL sentence is identical to that which would be produced by the
parse of the TL sentence.  
Transfer based approaches, in contrast, do
not assume identity between any part of the SL structure and TL
structure, but they differ with respect to the sort of
correspondences which can be described.  
Just as grammar
rules expressed in a unification-based formalism
can be regarded non-procedurally as constraints on well-formed
monolingual structures, transfer rules or translation links
can be regarded as constraints
on the mappings between source and target structures which are
translation equivalent. 
The particular approach to translation which we are exploring is
heavily lexicalist in orientation, although it is also possible to incorporate
non-lexical constraints.


We define lexical translation equivalence in terms of cross-linguistic
links, {\em tlinks}, stated in terms of lexical entries in the monolingual
lexicons. Because tlinks are defined
in terms of inheritance from lexical entries and rules,
translation equivalence can be stated through direct reference to
properties of word syntax and semantics as they appear in the
monolingual lexical descriptions. The sharing of information
structures between tlinks and the lexicons provides an
efficient and linguistically motivated way of characterizing classes
of lexical correspondence across languages, promotes compactness in
the description of translation equivalence and ensures that the
multilingual and monolingual components are compatible. 

The LRL is designed to be a general representation language, capable
of encoding a variety of linguistic approaches,
following the same sort of philosophy as PATR-II (Shieber, 1986).
This flexibility also applies to the tlink mechanism.
In the LRL the encoding of a linguistic theory is expressed in the
type system.
In the case of the type system adopted for work on ACQUILEX,
an important element of our approach is the
adoption of a common system of types to encode syntactic and semantic
properties of lexical items in the four languages investigated within
the project. 
This ensures compatibility of representation without ruling out language
specific parametrisation where necessary.
Such a practice is particularly effective in the treatment
of translation mismatches --- see below and Sanfilippo {\it et al.}~(1992). 
One other main feature of our approach to representation
is our emphasis on
representing detailed information about lexical semantics.
Again, this is justified by monolingual considerations, but we exploit
lexical semantic information in the multilingual aspects of 
representation.

\section{Monolingual representation}

Our approach to monolingual lexical representation involves combining
syntactic, formal semantic and lexical semantic information in a
single lexical entry. Lexical entries are represented as {\it typed
feature structures} (FSs) where syntactic and semantic information is
expressed in terms of attribute-value pairs. Consider, for example,
the lexical entry for the noun sense for {\it (drinking) 
chocolate}, shown below,
which was derived automatically from the {\it Longman Dictionary of
Contemporary English} (LDOCE; Procter, 1978). 
Here {\bf lex-noun-sign}
is a type which specifies syntactic and semantic properties of nouns
and {\tt drink\_L\_2\_1} is the name for the lexical entry from which
a value for the {\sc qualia} attribute is inherited by default
(\verb+<+).
The remainder of the entry specifies the MRD source.
\begin{verbatim}
chocolate_L_1_4
lex-noun-sign
<SENSE-ID : DICTIONARY> = "LDOCE"
<SENSE-ID : HOMONYM-NO> = "1"
<SENSE-ID : SENSE-NO> = "4"
<SENSE-ID : LDB-ENTRY-NO> = "5902"
<QUALIA> < drink_L_2_1 <QUALIA> .
\end{verbatim} 
When expanded, the lexical description above will yield a 
FS containing
syntactic and semantic information, as partially shown in AVM notation in
Figure~\ref{chocolate}. (Throughout this paper we will use bold font for types
and capitals for features.  In the AVM notation, reentrancy is indicated by
integer tags, angle brackets denote list structures and the boxes indicate parts
of the FS which are not shown completely.)
\begin{figure*}
{\tiny
\centering
$\avmplus{\att{lex-noun-sign}\\
         \attvaltyp{ORTH}{chocolate}\\
         \attvalshrunktyp{CAT}{noun-cat}\\
         \attval{SEM}{\avmplus{\att{unary-formula-entity-arg1}\\
                  \attvaltyp{PRED}{chocolate\_L\_1\_4}\\
                  \attvaltyp{ARG1}{entity}}}\\
         \attvalshrunktyp{SENSE-ID}{sense-id}\\
         \attval{QUALIA}{\avmplus{\att{c\_art\_subst}\\
                  \attval{PURPOSE}{\avmplus{\att{formula}\\
                                    \attvaltyp{PRED}{drink\_L\_1\_1}}}\\
                  \attvaltyp{PHYSICAL\_STATE}{liquid\_a}\\
                  \attvalshrunktyp{CONSTITUENCY}{constituency}\\
                  \attval{FORM}{\avmplus{\att{physform}\\
                           \attvaltyp{SHAPE}{non-individuated}}}}}}$

}
\caption{Lexical entry for (drinking) {\it chocolate}}
\label{chocolate}
\end{figure*} 
Our use of type inheritance and default inheritance allows a compact
lexical entry to be expanded into a detailed FS. This makes it
possible to avoid redefining the same information structures, thus
reducing a great deal of redundancy in the specification of word
forms.  The syntactic and formal semantic portions of the lexical sign
are relatively conventional, however we also encode detailed lexical
semantic information, the {\sc qualia} structure,
in a way which is loosely based on
Pustejovsky's work (e.g.~Pustejovsky, 1991).
The representation language used for lexical
description is also employed for grammar and
lexical rules, which are expressed as typed feature structures
describing relationships between two or more signs (FS
descriptions of single words or phrases).
We will use a simplified version of the type system for the examples
in this paper for ease of exposition.  

\section{Representation of translation equivalence}

Tlinks can be viewed as information structures describing ways in
which input lexical entries from the source and target languages are
mapped into output translation-equivalent pairs. All
tlinks are defined as FSs of type {\bf tlink} which relates the source
FS (SFS) and the target FS  (TFS) which are both of type {\bf rule}:
\begin{verbatim}
tlink (top)
< SFS > = rule
< TFS > = rule.
\end{verbatim}
Minimally, a rule establishes a correspondence between an input sign
(1) and an output sign (0):
\begin{verbatim}
rule (top)
< 0 > = sign
< 1 > = sign.
\end{verbatim} 
The rule-inputs of tlinks are meant to be instantiated by FS representations of
word senses in the source and target languages; rule-outputs provide the
translation equivalence.  Thus, when the FSs at the end of the paths \verb+< SFS
: 1 >+  and \verb+< TFS : 1 >+ are instantiated by lexical entries, the FSs at
the end of the paths \verb+< SFS : 0 >+ and \verb+< TFS : 0 >+ are
defined to be translation equivalent.\footnote{Tlinks are both
symmetrical and reversible; we use the terminology source,
target, input and output solely for ease of exposition.}
This level of indirection is crucial in expressing translation mismatches. 
Tlinks can be regarded as generating new FSs; given a FS in one
language, and an appropriate tlink, unification with the FS at the end
of the appropriate path, the {\em SL output},
(e.g. \verb+< SFS : 0 >+ ) in the tlink, will
result in the FS at the end of the other output path (the {\em TL output})
being returned (e.g. \verb+< TFS : 0 >+ ). 

The concept of translation equivalence is constrained by defining an
inheritance network of tlink types encoding generalisations relative
to classes of crosslinguistic equivalences. The commonest and simplest
cases of translation equivalence can be represented as {\bf
simple-tlink}s.
\eject
\begin{verbatim}
simple-tlink (tlink)
< SFS : 0 > = < SFS : 1 >
< TFS : 0 > = < TFS : 1 >
< SFS : 0 : SEM : ARG1 > = 
   < TFS : 0 : SEM : ARG1 >.
\end{verbatim}
A {\bf simple-tlink} is applicable in the case where two lexical
entries which denote single place predicates (nouns etc) are
straightforwardly translation equivalent, without any transformation
being necessary. 
The semantic variables
relative to the semantic entity described by two output structures are
specified to be identical.
This variable equivalence is the basis for the use of 
tlinks within unification based approaches to MT
and is crucial in expressing translation mismatches such
as thematic divergence and head switching (Sanfilippo {\it et al.},
1992).   The particular paths equated will depend on the way that the
type system is used to encode semantic structure --- in the examples which
follow a simplified encoding has been used for ease of exposition.

For example, assuming that the LDOCE sense
{\it chocolate} 1 4, is translation equivalent to the Van Dale {\it
chocolade} 0 2, we would have  the tlink:
\begin{verbatim}
simple-tlink
< SFS : 1 > <= chocolate_L_1_4 <>
< TFS : 1 > <= chocolade_V_0_2 <>.
\end{verbatim}
where \verb+<=+, indicates non-default inheritance from a named FS
(the particular lexical entries). This tlink will yield the English
entry as output when the Dutch entry is supplied as input and vice
versa. The `syntactically sugared' notation for this tlink, which will
be used in subsequent examples, 
is:
\begin{verbatim}
chocolate_L_1_4 / chocolade_V_0_2
simple-tlink.
\end{verbatim} 
For intransitive verbs we will make use of a slightly different tlink:
\begin{verbatim}
iverb-tlink (tlink)
< SFS : 0 > = < SFS : 1 >
< TFS : 0 > = < TFS : 1 >
< SFS : 1 > = iverb-sign
< TFS : 1 > = iverb-sign
< SFS : 0 : SEM : ARG2 > = 
  < TFS : 0 : SEM : ARG2 >.
\end{verbatim}
For example the tlink for `English:{\it fly} $\approx$ Italian:{\it
volare}'  would be:
\begin{verbatim}
fly_L_1_1 / volare_G_0_1
iverb-tlink.
\end{verbatim}
The expanded version of this tlink is shown in Figure~\ref{fly}
where the type {\bf move-manner} specifies the thematic functionality of the
subject participant as involving movement with manner of motion expressed
--- see Sanfilippo (in press) for details about the representation of verb
semantics adopted.

\begin{figure*}
{\tiny
\centering
$\avmplus{
 \att{iverb-tlink}\\
 \attval{SFS}{\avmplus{\att{rule}\\
              \attval{0}{\ind{0}}\avmplus{\att{iverb-sign}\\
                                       \attvaltlist{ORTH}{flies}\\[2mm]
                                       \attvalshrunktyp{CAT}{ivcat} % \avmplus{\att{ivcat}\\
                                                      %   \attvaltyp{HEAD}{verb}\\
                                                      %   \attvaltlist{SUBCAT}{np-sign} }}
                                                     \\[2mm]              
                                       \attval{SEM}{\avmplus{\att{ivsem}\\
                                                         \attvaltyp{PRED}{and}\\
                                                        \attval{ARG1}{\avmplus{\att{vformula}\\
                                                                 \attvaltyp{PRED}{fly\_L\_1\_1}\\
                                                                 \attvaltyp{ARG1}{$\ind{2}$process}}}\\
                                                       \attval{ARG2}{\ind{1}\ \avmplus{\att{thformula}\\ 
                                                                 \attvaltyp{PRED}{move-manner}\\
                                                                 \attval{ARG1}{\ind{2}}\\
                                                                 \attvaltyp{ARG2}{obj} }} }} }\\
             \attval{1}{\ind{0}}}}\\
 \attval{TFS}{\avmplus{\att{rule}\\
              \attval{0}{\ind{3}}\avmplus{\att{iverb-sign}\\
                                  \attvaltlist{ORTH}{vola}\\[2mm]
                                  \attvalshrunktyp{CAT}{ivcat} % \avmplus{\att{ivcat}\\
                                                      %   \attvaltyp{HEAD}{verb}\\
                                                      %   \attvaltlist{SUBCAT}{np-sign} }}
                                                     \\[2mm]                  
                                  \attval{SEM}{\avmplus{\att{ivsem}\\
                                                \attvaltyp{PRED}{and}\\
                                                \attval{ARG1}{\avmplus{\att{vformula}\\
                                                              \attvaltyp{PRED}{volare\_G\_0\_1}\\
                                                              \attval{ARG1}{\ind{2}}}}\\
                                                              \attval{ARG2}{\ind{1}}}}}\\
              \attval{1}{\ind{3}}}}}$

}
\caption{Expanded tlink for `English:{\it fly} $\approx$ Italian:{\it volare}'}
\label{fly}
\end{figure*}

Tlinks can be viewed as constraining the relationship between
structures in the source and target languages.  If the SL output of
some tlink
unifies with some part of the structure that results from parsing a
sentence, then the structure given to the TL sentence is constrained
to include the TL output of that instantiated tlink.
Consider, for example, the structure in Figure~\ref{sl} which is a
simplified version of the analysis for the sentence {\it Tweety flies}
(ignoring morphology). Here we have assumed an HPSG-like treatment: note that
the sign is equivalent to an entire parse tree, in that it contains
the signs that have been combined to form the sentence sign itself, and
that the arguments of the lexical signs have been coindexed as a
result of the parse.  The SL output
of the tlink shown in Figure~\ref{fly} can be
unified with the instantiated lexical sign for the verb (i.e. 
% \verb+< DTRS : HEAD-DTR : DTRS : HEAD-DTR >+ 
the FS value for the path \verb+< DTRS : HEAD-DTR >+ 
in Figure~\ref{sl}) and therefore the TL structure is constrained to contain 
a structure which is subsumed by the TL output of the tlink. 
The same applies to the tlink relating the SL and TL subject noun
phrases which in this case expresses simple identity. Because the variables
corresponding to the arguments of the lexical sign are coindexed in the SL
structure, and the tlinks contain statements of argument equivalence, the
arguments of the lexical signs in the TL structure will also be constrained to
be identical.  Given the constraints of the monolingual grammar, plus the
additional assumption that no additional predicates may be inserted, this is
sufficient information to constrain the TL structure for the complete sentence
to that shown in Figure~\ref{tl}.

\begin{figure*}
{\tiny
\centering
$\avmplus{
 \att{s-sign}\\
 \attvaltlist{ORTH}{$\ind{1}$ Tweety, $\ind{2}$ flies}\\[2mm]
 \attvalshrunktyp{CAT}{scat}\\[2mm]
 \attval{SEM}{\avmplus{\attvaltyp{PRED}{and}\\
                   \attval{ARG1}{\ind{3}\ \avmplus{\att{ivsem}\\
                                             \attvaltyp{PRED}{and}\\
                                             \attval{ARG1}
                                                    {\avmplus{\att{vformula}\\
                                                              \attvaltyp{PRED}{fly\_L\_1\_1}\\
                                                             \attvaltyp{ARG1}{$\ind{4}$process}}}\\
                                             \attval{ARG2}
                                                    {\avmplus{\att{thformula}\\ 
                                                              \attvaltyp{PRED}{move-manner}\\
                                                              \attval{ARG1}{\ind{4}}\\
                                                              \attvaltyp{ARG2}{$\ind{5}$\ obj}}} }}\\[4mm]
                   \attval{ARG2}{\ind{6}\ \avmplus{\att{npsem}\\
                                                  \attvaltyp{PRED}{tweety} \\
                                                  \attval{ARG1}{\ind{5}}}} }} \\[5mm]
\attval{DTRS}
       {\avmplus{\att{head-comp-struc}\\
                 \attval{HEAD-DTR} % DTRS$\mid$HEAD-DTR
                        {\avmplus{\att{iverb-sign}\\
                                  \attvaltlist{ORTH}{$\ind{2}$}\\
                                  \attval{CAT}{\avmplus{\att{ivcat}\\
                                                        \attvaltyp{HEAD}{verb}\\
                                                        \attvaltlist{SUBCAT}{$\ind{7}$} }}\\[4mm]
                                  \attval{SEM}{\ind{3}} }} \\
                 \attvallist{COMP-DTRS}{\ind{7} 
                                        \avmplus{\att{np-sign}\\ 
                                                 \attvaltlist{ORTH}{$\ind{1}$}\\[1mm]
                                                 \attvalshrunktyp{CAT}{npcat}\\[1mm]
                                                 \attval{SEM}{\ind{6}}}} }} }$

}

\caption{Structure resulting after parse of source language sentence}
\label{sl}
\end{figure*}

\begin{figure*}
{\tiny
\centering
$\avmplus{\att{s-sign}\\
         \attvaltlist{ORTH}{Tweety, vola} \\[2mm]
         \attvalshrunktyp{CAT}{scat}\\[2mm]
        \attval{SEM}{\avmplus{\attvaltyp{PRED}{and}\\
                   \attval{ARG1}{\avmplus{\att{ivsem}\\
                                             \attvaltyp{PRED}{and}\\
                                             \attval{ARG1}
                                                    {\avmplus{\att{vformula}\\
                                                              \attvaltyp{PRED}{volare\_G\_0\_1}\\
                                                             \attvaltyp{ARG1}{$\ind{4}$process}}}\\
                                             \attval{ARG2}
                                                    {\ind{1}\ 
                                                     \avmplus{\att{thformula}\\ 
                                                              \attvaltyp{PRED}{move-manner}\\
                                                              \attval{ARG1}{\ind{4}}\\
                                                              \attvaltyp{ARG2}{$\ind{5}$\ obj}}} }}\\[4mm]
                   \attval{ARG2}{\avmplus{\att{npsem}\\
                                                  \attvaltyp{PRED}{tweety} \\
                                                  \attval{ARG1}{\ind{5}}}} }}
\\[6mm]
         \attvalshrunktyp{DTRS}{head-comp-struc}}$

}
\caption{Italian translation for {\it Tweety flies}}
\label{tl}
\end{figure*}

\eject
We have implemented a general constraint-based system 
which basically works by attempting to match tlinks against
the SL structure resulting from a parse and then generating from the
``bag'' of feature structures in the TL which result.   This has
strong similarities with the Shake-and-Bake approach to translation
(Beaven, 1992; Whitelock, 1992). However, non-lexical
constraints may also be incorporated. For example, we might wish to ensure that
the speech act type represented by the SL and TL sentences (e.g. {\it wh}-question,
{\it yes/no}-question or imperative) match. In our MT system, this can be
done by stating a constraint on the {\bf s-sign}s, without any reference to
lexical entries. The system is intended for testing tlinks, rather than as a
practical approach to MT, and it is therefore designed for generality rather than
efficiency. Further details of this system, including descriptions of how it
handles multiple possibilities and phrasal equivalences, are given in
Copestake (1993).  
We will consider the relationship to more practical approaches to MT
in the conclusion, and concentrate on the lexical aspects here.

\section{More complex translation links}

Some restrictions on translation can be expressed by making the target
or source FSs of tlinks more specific. For example, we can define a
type {\bf human-tlink} and state as a constraint that the values for
the {\sc sex} feature must be the same in the translation equivalent
feature structures:
\begin{verbatim}
human-tlink (simple-tlink)
< SFS : 0 : QUALIA : SEX > = 
   < TFS : 0 : QUALIA : SEX >.
\end{verbatim}
A tlink of this type would be suitable for establishing equivalences
such as that between the word {\it teacher} in English and its
translation in Spanish as either {\it maestro} or {\it maestra}. The
restriction that {\it maestro} denotes a male teacher and {\it
maestra} a female one --- i.e. the values for \verb+< QUALIA : SEX >+ in
the FS descriptions for the two words are {\bf male} and {\bf female}
respectively --- follows from the use of tlinks of type {\bf
human-tlink} to relate {\it teacher} with {\it maestro} and {\it maestra}:
\begin{verbatim}
teacher / maestro :
human-tlink.

teacher / maestra :
human-tlink.
\end{verbatim}
The path equation induced by the {\bf human-tlink} will ensure that
translation equivalent words denote individuals of the same (natural)
gender.\footnote{As 
we will see later, the morphological generalisation involved in
this example can
be captured, making it unnecessary to manually specify both tlinks.}
The use of a common type system makes it easier to express such
relationships, although it is not essential, provided that comparable
information is encoded in both monolingual representations.

Somewhat rarer and more complex cases of linking arise when a
translation mismatch involves transformations operating on FSs.  For
example, the equivalence class resulting from translation pairs such
as `English:{\it furniture} $\approx$ Spanish:{\it muebles}' can be
represented by establishing a link between a word and its translation
in the plural form. In this case the equivalence holds between a basic
lexical entry and a lexical entry after rule application. In our
approach, this behaviour can be modelled straightforwardly, by
instantiating one half of the tlink with the appropriate lexical rule.
This is shown in the tlink below where the
target-language side (\verb+< TFS >+) inherits from the FS
representation for the plural rule whose input is the tlink's
target input ({\tt mueble}) and output is the tlink's
target output.
\begin{verbatim}
furniture / mueble :
tlink
< SFS : 0 > = < SFS : 1 >
< TFS > <= plural <>.
\end{verbatim}
Note that the lexical/morphological rule for plural
formation used in the tlink is needed anyway for use in
parsing/generation, so that its use in tlinks does not involve
introduction of elements other than those needed in the monolingual
grammars. The tlink above can be represented figuratively as shown in
Figure~\ref{furniture} 
where unlabelled arrows indicate token identity between FSs.
\begin{figure*}
\begin{center}
\begin{picture}(105,25)(-5, 15)
\thinlines    \put(5,25){\makebox(0,0){SFS1}}
              \put(5,18){\makebox(0,0){furniture}}
%             \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){furniture}}
              \put(50,35){\makebox(0,0){\bf 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){muebles}}
              \put(77,30){\makebox(0,0){plural}}
              \put(90,25){\vector(-1,0){25}}
              \put(95,25){\makebox(0,0){TFS1}}
              \put(95,18){\makebox(0,0){mueble}}
\end{picture}
\end{center}
\caption{Figurative representation of translation link relating {\it
furniture} and {\it muebles}}
\label{furniture}
\end{figure*}
Since the singular form {\it mueble} would not unify with the
feature structure at the end of the output path \verb+< TFS : 0 >+, a
translation of {\it mueble} as {\it furniture} would not be generated
and another tlink would be required to relate the two words when translating
a phrase such as {\it a piece of furniture}. 
%In general, 
%lexical gaps can be represented by relating a lexical item to a feature
%structure representing a phrase.  Such a phrase need not be fully instantiated. 
Such a tlink would relate a lexical item to a feature structure representing a
partially specified phrase whose full instantiation is established contextually.
For example, we can specify that some form of individuating phrase is  required
rather than providing a literal translation of {\it mueble} as {\it a piece of
furniture}; this would allow the possibility of {\it item} as well as {\it
piece}. We can also allow for the possibility of material being interpolated
(e.g. {\it piece of English furniture}).  Further details are given in Copestake
(1993).

In general, tlinks between a lexical item and a phrase are useful in the
treatment of lexical gaps due to a lack of correspondence in sense-extension
productivity. For example, a considerable number of verbs can occur as either
inchoatives or causatives in English but can only be used as inchoatives in
Italian. This phenomenon is particularly recurrent with manner-of-motion verbs,
as show in the examples below where the lack of a causative equivalent for verbs
such as {\it fly, run, march} in Italian is resolved by combining inchoative
{\it volare, correre, marciare} with the causative verb {\it fare}:  
\begin{ex}
\begin{subex}
Jamie {\it flew} his kite \\
Jamie {\it fece volare} l'acquilone
\end{subex}
\begin{subex}
The trainer {\it ran} the filly at Newmarket\\ 
L'allenatore {\it fece correre} la puledra a Newmarket
\end{subex}
\begin{subex}
The general {\it marched} the soldiers \\ 
Il Generale {\it fece marciare} i soldati
\end{subex}
\end{ex}
The generalization underlying such equivalence can be expressed in terms of diverging lexicalization
strategies (Talmy, 1985): English verbs such as {\it fly, run, march} are allowed to
integrate causation, while their Italian equivalents require that causation be expressed separately.
Using the tlink mechanism, this divergence can be expressed as an equivalence between a transitive
verb and a verb phrase consisting of two verbs (the head and complement daughters) as shown in
Figure~\ref{tvvp} with the type {\bf tverb-vp-tlink}.     
\begin{figure*} 
{\tiny 
\centering 
$\avmplus{\att{tverb-vp-tlink}\\
 \attvalshrunktyp{SFS$\mid$0}{tverb-sign} \\[2mm]
 \attval{TFS$\mid$0}{\avmplus{\att{vp-sign}\\
                              \attval{DTRS}{\avmplus{\attvalshrunktyp{HEAD-DTR}
                                                                     {vxcomp-sign} \\[2mm]
                                                     \attval{COMP-DTRS}
                                                            {\langle\ \boxvaluebold{iverb-sign}
                                                         \ \rangle} }}}} }$

}  
\caption{Tlink between a transitive verb ({\bf tverb-sign}) and a verb phrase ({\bf vp-sign})
consisting of a matrix predicate ({\bf vxcomp-sign}) and its verbal complement ({\bf iverb-sign}) }
\label{tvvp}
\end{figure*}
The equivalence between the two lexicalization strategies can be semantically expressed by stating
that  
\begin{itemize}
\item an English verb describing caused motion such as {\it fly} corresponds to Italian
causative {\it fare} plus the inchoative Italian equivalent of the English motion verb ({\it
volare}), and
\item the agent and theme roles across the source and target sides of the tlink are identical.
\end{itemize}
In Figure~\ref{flyfarevolare}, this is indicated in the type 
{\bf tverb-vp-caused\_motion-tlink} 
where
the type {\bf tverb-vp-tlink} (the parent tlink) is augmented with specifications concerning verb
semantics. The role types {\bf cause} and {\bf move-manner} which characterize the thematic
functionality  of agent and theme participants restrict the application of the tlink to verbs
expressing caused motion with manner expressed; the identity of the two roles across the source and
target sides of the tlink (encoded by the integer tags $\ind{2}, \ind{3}$) specifies the equivalence
in lexicalization strategies.
 \begin{figure*}
{\tiny
\centering
$ 
\avmplus{\att{tverb-vp-caused\_motion-tlink}\\
\attval{SFS$\mid$0}
       {\avmplus{\att{iverb-sign}\\
                % \attvaltlist{ORTH}{fly}\\
                 \attvalshrunktyp{CAT}{ivcat}\\
                 \attval{SEM}{\avmplus{\att{tvsem}\\
                                       \attvaltyp{PRED}{and}\\
                                       \attvalshrunktyp{ARG1}{vformula}\\
                                             % {\avmplus{\att{vformula}\\
                                             %           \attvaltyp{PRED}{fly\_L\_1\_1}\\
                                             %           \attvaltyp{ARG1}{$\ind{1}$process} }} \\
                                       \attval{ARG2}
                                              {\avmplus{\att{binformula}\\
                                                        \attvaltyp{PRED}{and}\\
                                                        \attval{ARG1}
                                                               {\ind{2}
                                                                \avmplus{\att{thformula}\\ 
                                                                         \attvaltyp{PRED}{cause}\\
                                                                         \attval{ARG1}{\ind{1}}\\
                                                                         \attvaltyp{ARG2}{obj}}}\\[6mm]
                                                        \attval{ARG2}
                                                               {\ind{3}
                                                                \avmplus{\att{thformula}\\ 
                                                                         \attvaltyp{PRED}{move-manner}\\
                                                                         \attval{ARG1}{\ind{1}}\\
                                                                         \attvaltyp{ARG2}{obj} }} }} }}
        }}\\[8mm]

\attval{TFS$\mid$0}
       {\avmplus{\att{vp-sign}\\
                 \attvaltlist{ORTH}{fare, $\ind{5}$}\\[2mm]
                 \attvalshrunktyp{CAT}{vpcat}\\[2mm]                 
                 \attval{SEM}{\avmplus{\att{vxcompsem}\\
                                       \attvaltyp{PRED}{and}\\
                                       \attval{ARG1}
                                              {\avmplus{\att{vxcompformula}\\
                                                        \attvaltyp{PRED}{fare}\\
                                                        \attvaltyp{ARG1}{eve}\\
                                                        \attval{ARG1}{\ind{1}} }} \\[6mm]
                                       \attval{ARG2}
                                               {\avmplus{\att{binformula}\\
                                                         \attvaltyp{PRED}{and}\\
                                                         \attval{ARG1}{\ind{2}}\\
                                                         \attval{ARG2}{\ind{4}} }} }} \\
                 \attval{DTRS$\mid$COMP-DTRS}
                        {\avmplus{\attvaltlist{ORTH}{$\ind{5}$}\\[2mm]
                                  \attval{SEM}{\ind{4}
                                               \avmplus{\att{ivsem}\\
                                                        \attvaltyp{PRED}{and}\\
                                                        \attvalshrunktyp{ARG1}{vformula} \\[2mm]  
                                                                     %{\avmplus{\att{vformula}\\
                                                                     % \attvaltyp{PRED}{volare\_G\_0\_1}\\
                                                                     % \attvaltyp{ARG1}{$\ind{1}$process}}}\\[4mm]
                                                         \attval{ARG2}{\ind{3}} }} }} }}
 } $ 

}
\caption{Tlink for equivalences such as `English:{\it fly} $\approx$ Italian:{\it fare volare}'. 
The `cause' (agent) and `move-manner' (theme) roles of the source-language verb correspond to the 
`cause' role of {\it fare} and the `move-manner' role of the other target language verb. }
\label{flyfarevolare} 
\end{figure*}

In some cases the existence of a tlink between two lexical items
implies a further translation relationship.  For example, in English,
there is a regular sense extension such that a word used primarily to
denote an animal can also be used to denote the meat of that animal
(e.g. {\it lamb}, {\it chicken}, see Copestake and Briscoe (1992)).  A
similar sense extension rule applies to Italian (\"{O}stling 1991) but
in Dutch a compound is generally used ({\it lam}, {\it lamvlees}).
The correspondence between the two processes (e.g. sense extension in
English and compounding in Dutch) and their import on translation
equivalence can be expressed by means of a {\em tlink-rule} which allows
transformations to be defined for tlinks. For example, we can
automatically generate the relationship between the FS representing
the animal sense of {\it lamb} ({\tt lamb\_1}) and {\tt
lamvlees} by transforming the simple tlink between the FS representing
the meat sense of {\it lamb} ({\tt lamb\_2}) and
{\tt lam} into a new tlink where the source input FS results from
applying the sense extension rule `animal-grinding' to {\tt lamb\_1}
and the source input FS results from compounding {\it lam} with {\it
vlees}, as shown diagrammatically
in Figure~\ref{lamb}.  Since we are just making use of 
the monolingual sense extension mechanism here we can rely on that to 
handle cases where the sense extension is blocked (e.g. {\it pig}).
It does
not necessarily matter for translation purposes
whether the lexical rule can fully predict the effects of the sense extension;
even if it is used to encode the
regular aspects of the relationship between two existing lexicalised items, 
an appropriate translation link will be generated
if the monolingual processes are sufficiently similar.
\begin{figure*}
\begin{center}
\begin{picture}(140,80)(-20,-40)
\thinlines    \put(5,25){\makebox(0,0){SFS1}}
              \put(5,18){\makebox(0,0){lamb1}}
              \put(10,25){\vector(1,0){25}}
              \put(40,25){\makebox(0,0){SFS0}}
              \put(40,18){\makebox(0,0){lamb1}}
              \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){lam}}
              \put(90,25){\vector(-1,0){25}}
              \put(95,25){\makebox(0,0){TFS1}}
              \put(95,18){\makebox(0,0){lam}}

              \put(5,15){\vector(0,-1){35}}
              \put(-10,0){\makebox(0,0){\shortstack{animal-\\ grinding}}}
              \put(95,15){\vector(0,-1){35}}
              \put(110,0){\makebox(0,0){\shortstack{vlees\\ compounding}}}

              \put(5,-25){\makebox(0,0){SFS1}}
              \put(5,-32){\makebox(0,0){lamb2}}
              \put(10,-25){\vector(1,0){25}}
              \put(40,-25){\makebox(0,0){SFS0}}
              \put(40,-32){\makebox(0,0){lamb2}}
              \put(50,-15){\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,-32){\makebox(0,0){lamvlees}}
              \put(90,-25){\vector(-1,0){25}}
              \put(95,-25){\makebox(0,0){TFS1}}
              \put(95,-32){\makebox(0,0){lamvlees}}
\end{picture}
\end{center}
\caption{Tlink rule relating animal and meat senses of {\it lamb} to
{\it lam} and {\it lambvlees}}
\label{lamb}
\end{figure*}

There are many examples of such correspondences; for example the
English sense extension between trees and their fruits ({\it pear}\/
etc) is mirrored in Italian with a gender distinction; the trees are
masculine but the fruits feminine ({\it pero}, {\it pera}).  
In fact,
our earlier examples of {\it muebles} $\approx$ {\it furniture} and {\it
maestro/a} $\approx$ {\it teacher} can also be described more generally.
A tlink-rule can be defined for human-denoting nouns so that given the 
equivalence between two entries of one gender 
the tlink for the other gender can be produced.
Since gender in English is normally unmarked, one instantiation of
this is with a `null lexical rule', which states equality between
two FSs.
Thus given the tlink between
{\it teacher} and {\it maestra} we can automatically generate that
between {\it teacher} and {\it maestro} (see Figure~\ref{teacher}).
\begin{figure*}
\begin{center}
\begin{picture}(140,80)(-20,-40)
\thinlines    \put(5,25){\makebox(0,0){SFS1}}
              \put(5,18){\makebox(0,0){teacher}}
              \put(10,25){\vector(1,0){25}}
              \put(40,25){\makebox(0,0){SFS0}}
              \put(40,18){\makebox(0,0){\shortstack{teacher\\ +male}}}
              \put(50,35){\makebox(0,0){\bf human-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){maestro}}
              \put(90,25){\vector(-1,0){25}}
              \put(95,25){\makebox(0,0){TFS1}}
              \put(95,18){\makebox(0,0){maestro}}

              \put(5,15){\vector(0,-1){35}}
              \put(-10,0){\makebox(0,0){equality}}
              \put(95,15){\vector(0,-1){35}}
              \put(110,0){\makebox(0,0){masc/fem}}

              \put(5,-25){\makebox(0,0){SFS1}}
              \put(5,-32){\makebox(0,0){teacher}}
              \put(10,-25){\vector(1,0){25}}
              \put(40,-25){\makebox(0,0){SFS0}}
              \put(40,-32){\makebox(0,0){\shortstack{teacher\\ +female}}}
              \put(50,-15){\makebox(0,0){\bf human-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){maestra}}
              \put(90,-25){\vector(-1,0){25}}
              \put(95,-25){\makebox(0,0){TFS1}}
              \put(95,-32){\makebox(0,0){maestra}}
\end{picture}
\end{center}
\caption{Tlink rule for {\it teacher} $\approx$ {\it maestro/a}}
\label{teacher}
\end{figure*}
The count-mass
discrepancy which is responsible for the need to invoke pluralisation
in the {\it muebles} $\approx$ {\it furniture} 
example can also be generalised. 
Rather than
manually specifying the translations of both the singular and plural
count nouns for every case
we can specify that the
translation equivalent of the singular noun will be some more complex
construction which individuates the mass noun.

As the examples above have shown, multilingual representation requires
access to detailed lexical semantic information. Such a requirement
does not imply provision of task specific information, since access to
detailed lexical semantics is also needed to achieve adequacy in
monolingual representation (see, for example, Briscoe {\it et al.},
1990; Copestake and Briscoe, 1992). Furthermore, similarity in lexical
semantic representation can be used to allow sense selection when
constructing tlinks semi-automatically from bilingual dictionaries
(Copestake {\it et al.}, 1992).  For example, given that a bilingual
dictionary gives {\it cioccolata}, {\it cioccolato} and {\it
cioccolatino} as the Italian translations of {\it chocolate}, we can
determine that {\tt chocolate\_L\_1\_4} translates as {\it cioccolata}
because of the comparatively close similarity between their lexical
semantic structures.

\section{Tlinks in machine translation}

General constraint resolution is not a realistic approach to
MT because of its computational intractability, but we can classify MT
systems in terms of such a model according to the sort of translation
constraints they assume, and how they control the process of
translation.  
For example in SRI's BCI system (Alshawi {\it et al.}, 1991) the
source language string is parsed and transfer is carried out on 
the (quasi-)logical form representation.  This produces a
quasi-logical form appropriate for the TL, which can be used to drive
a head-driven generator.
In contrast, the Shake-and-Bake approach (Whitelock,
1992; Beaven, 1992) relies on lexical transfer operating on lexical
signs which have had their variable instantiated as a result of
parsing.  
As in the BCI system, transfer operates before generation, but in
Shake-and-Bake generation is constrained by `bags' of instantiated
lexical entries rather than an ordered representation of the sentence
as a whole.  This has the advantage that there is no problem 
of potential mismatch between the monolingual grammars.
However it also means that
normal generation techniques are not
applicable; the algorithm actually used roughly involves
producing all possible orderings of transferred target language signs
and using standard parsing techniques to
determine which of these orderings meet the constraints of the target
language grammar.  
The ELU system (Estival {\it et al.},
1990) demonstrates a third approach; transfer
operates at both a lexical and a phrasal level, and 
transfer and generation are interwoven by making use of
transfer variables which indicate explicitly that parts of the
structure remain untranslated.  

Tlinks contain enough information to 
potentially be usable to derive the lexical
cross-linguistic component in any of these systems, because they all
make use of declarative representation techniques and maintain a
distinction between the monolingual grammars and the translation
relationships.
To make use of tlinks in the BCI system, for example, 
syntactic information would be ignored, for the ELU system,
uninstantiated portions of phrasal signs would be explicitly marked by
transfer variables.  
Tlinks could straightforwardly be used to drive 
the strongly lexicalist approach adopted by Shake-and-Bake, where all
correspondences are stated at the lexical level.  
Because tlinks contain very little additional information beyond that
found in the monolingual grammars and lexicons, we do not foresee that
the difficulties of exploiting the multilingual LKB in other systems
are significantly greater than those involved in the monolingual case.

\section*{References}
\newcommand{\book}[4]{\item #1 (#4) {\it #2,} #3.}
\newcommand{\booknoauthor}[3]{\item #1 (#3) {\it #2,}}
\newcommand{\forthedbook}[4]{\item #1 (eds.) (#4, forthcoming) {\it #2,} #3.}
\newcommand{\unpub}[4]{\item #1 (#4) {\it #2,} #3.}
\newcommand{\bookart}[7]{\item #1 (#7) `#2' in #5 (eds.), {\it #4,} #6, pp.~#3.}
\newcommand{\bookartnopp}[6]{\item #1 (#6) `#2' in #4 (eds.), {\it #3,} #5.}
\newcommand{\forthart}[6]{\item #1 (#6, forthcoming) `#2' in #4 (eds.),
{\it #3,} #5.}
\newcommand{\journart}[6]{\item #1 (#6) `#2', {\it #3,} {\it vol.#4,} #5.}
\newcommand{\procart}[6]{\item #1 (#6) `#2', {\it Proceedings of the
#3,} #4, pp.~#5.}
\newcommand{\forthprocart}[5]{\item #1 (#5) `#2', 
to appear in {\it Proceedings of the #3,} #4}
\newcommand{\procartnopp}[5]{\item #1 (#5) `#2', {\it Proceedings of the
#3,} #4.}

\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}

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

%\bookart{Briscoe, E.J.}
%    {Lexical issues in natural language processing}
%    {39--68}
%    {Natural language and speech}
%    {E. Klein and F. Veltman}
%    {Springer-Verlag}
%    {1991}

\procart{Briscoe, E. J., A. Copestake  and B. Boguraev}
    {Enjoy the paper: lexical semantics via lexicology}
    {13th International Conference on Computational Linguistics (COLING-90)}
    {Helsinki}
    {42--47}
    {1990}

\book{Briscoe, E.J., A.~Copestake and V.~de Paiva} 
{Inheritance, defaults and the lexicon}
     {Cambridge University Press, Cambridge, UK}
     {in press}

\procart{Copestake, A.}
{The ACQUILEX LKB: representation issues in semi-automatic
acquisition of large lexicons}
{3rd Conference on Applied Natural Language Processing (ANLP-92)}
{Trento, Italy}
{88--96}
{1992} 

\unpub{Copestake, A.}
{Constraints, tlinks and MT}
{ACQUILEX Working paper}
{1993}

\bookart{Copestake, A. and E.~J.~Briscoe}
     {Lexical operations in a unification based framework}
     {101--119}
     {Lexical Semantics and Knowledge
     Representation.  Proceedings of the first SIGLEX Workshop, 
     Berkeley, CA}
     {J. Pustejovsky and S. Bergler}
     {Springer-Verlag, Berlin}
     {1992}

\bookart{Copestake, A., B.~Jones, A.~Sanfilippo, H.~Rodriguez, P.~Vossen,
S.~Montemagni and E.~Marinai}
{Multilingual lexical representation}
{117--129}
{The (other) Cambridge ACQUILEX papers}
{A.~Sanfilippo}
{University of Cambridge Computer Laboratory. Technical Report No. 253}
{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}

\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}

\unpub{\"{O}stling, A.}
    {Sense extensions in the Italian food subset} 
    {ACQUILEX working paper, Dipartimento di Linguistica,
Universit\`{a} di Pisa}
    {1991}

\book{Procter, P. (ed)}
    {Longman Dictionary of Contemporary English}
    {Longman, London}
    {1978}

\journart{Pustejovsky, J.}
    {The generative lexicon}
    {Computational Linguistics}
    {17(4)}
    {409--441}
    {1991}

\bookartnopp{Sanfilippo, A.}
    {NLP Encoding of Lexical Knowledge}
    {Inheritance, defaults and the lexicon}
    {Briscoe, E.J., A.~Copestake and V.~de Paiva}
    {Cambridge University Press, Cambridge, UK}
    {in press}

\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}


\procart{Sanfilippo, A. and V. Poznanski}
{The Acquisition of Lexical Knowledge from Combined Machine-Readable
Dictionary Sources}
{3rd Conference on Applied Natural Language Processing (ANLP-92)}
{Trento, Italy}
{80--88}
{1992}

\book{Shieber, S.~M.}
{An Introduction to Unification-Based Approaches to Grammar}
{CSLI Lecture Notes 4, Stanford CA}
{1986}

\bookartnopp{Talmy, L.}
{Lexicalization Patterns: Semantic Structure in Lexical Form}
{Language Typology and Syntactic Description 3. Grammatical Categories
 and the Lexicon}
{T.~Shopen}
{Cambridge University Press}
{1985}

\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}
