An active bilingual lexicon for machine translation

An approach to tile Transfer phase of a Machine Translat ion system is presented, where the bilingual lexicon plays an active role, guiding Transfer by means of executable descriptions of word senses. The means for lexical sense specification are, however, general enough and can in principle apply to other system arthitectures, e.g. in tile Generation phase if Transfer is intentionally kept minimal. The active lexicon is the one and only systea~ componen t which is exposed to users and can serve to linguistically control Transfer effects. A unified approach to lexicon creation and maintenance is proposed, which contains means to gradually refine sense specification and tailor the definitions to specific text domains. The underlying linguistic principles, the nature of sense distinction required tot translation, and tilt: formal structure of the lexicon are discussed. I° h t t r o d u c t i o n While melbods of monolingual Analysis and Generation are also treated in other contexts, bilingual Transfer problems are hardly inw~stigated outside the context of Machine Translation. Research in Machine Translation can, in this case, make a specific contribution to Computat ional l,inguistics. The general issue here is tire formal representation of phrase structures and lexical units and t h e methodology for specifying transformations between these representations in two (or more) languages. The role of tile bilingual lexicon in the Transfer activity, attd its power to assist in the resolution of mapping problems, is a key element. In tills paper we present an approach to the formal representation of bilingual lexical knowledge and to the way this knowledge is incorporated into the translation process. In section 2 we describe the role and place of the bilingual lexicon in the translation process, present the concept of executable descriptions of word senses as lexical definitions, and discuss some aspects of practical usage. Our approach to the sense distinction required for translation, which is different f rom monolingual sense distinction, is discussed in section 3. In section 4 we make a few methodological comments , arguing that wha! is often portrayed as the ideal Transfer-based architecture, is not the only, and not necessarily the best way to achieve modularity and save work. Section 5 contains a formal definition of the lexicon specification language with some discussion of its features and the intended restrictions on the power given to the lexicographer. Finally, an additional example is given in detail in section 6. This work has been carried out as part of the M E N T O R project, where several groups in European IBM Scientific Centers are collaborating on M(A)T research. The approach presented here has been developed and prototyped by the group in Ilaifa, Israel, as part of the proposal for tire design of Transfer-related operations. The examples below involve translations from English into I lebrew. We thank our colleagues Danit Ben-Ari, Esther Bentur and Maria Vilkuna for their contributions and comments . 2. "lhe Role and Content of the Bilingt, al Lexicon [Cullingford 87] describes an MT system which is purely lexicon driven. Il ls system follows the Conceptual Processing model, and is not Transfer-based, hence the emphasis there is on deep Analysis and Generation. Many other systems distinguish between I.exical Transfer and Structural Transfer, but they take different approaches to the actual separation of these two sub-processes. In the work reported here, an attempt was made to strictly separate lexicon-driven selection of target language equivalents from the global mapping of syntactic structures in the SIA into those of tile 'I'I~ (cf. [Biewer 85]). ' lhe lexicon lookup phase, which takes place before phrase structure transformations, gets as its input the internal data representation provided by the SL parser (PEG [Jensen 86], in out prototype). The terminal nodes (leaves of a parse tree) are searched in a pre-defined order for certain parts of speech. For each word in turn a target equivalent is selected from the bilingual lexicon and attached to the corresponding node in the parse. Features may also be added to other affected nodes. f lowever, no structural modifications are made. Structural t ransformations are carried out as an independent sub-process, upon completion of tire bilingual lexical phase, and are not discussed in this paper. Since in many cases, and ill fact for most verbs, several alternative translations exist, tile selection is done by texical differentiation rules. These rules refer to the syntactic environment of tile word in the parse tree and to a limited number of semantic features. The rules can access any node and attribute identified by the parser. Given that the rules are stated in terms of the SL phrase structure, it seems more natural to apply them as close to Analysis as possible. Nevertheless, the sense distinction cannot be done as part of SL Analysis itself, as in many cases it depends on factors which may vary from one TL to another. The sub-process of bilingual lexical substitution proceeds unidirectionally. No iterations take place for any given phrase. In some cases this may require extensive searching of' the phrase The filllowing abbreviation~ are u~ed throughout thi~ pal~er: S L =: Sou rce L a n g u a g e , B L = BiLingua l , T L = T a r g e t Language