Extended Dependency Structures and their Formal Interpretation

Extended Dependency Structures and their Formal Interpretation Marc Dymetman Max Coppcrman Rank Xerox Research Centre 6, therein de Maupertuis, Meylan 38240, France { dyme tman, copperman} @xerox. fr Abstract We describe two "semantically-oriented" dependency-structure formalisms, t J-forms and S-forms. U-forms have heen previously used in nmchine translation as interlingual representations, hut without being provided with a formal interpretation. S-forms, which we introduce in this paper, are a scoped vet sion of U-fnrms, and we define a composi- tional semantics mechanism for them. Two types of semantic composition are basic: complement incorlgoration and modifier in- corporation. Binding of variables is done at tire time of incorporation, permitting tnttch [lexibility in composition order and a simple account of the semantic effects of permuting several incorporations. 1 INTRODUCTION U-fi)rms (Unscoped dependency form) arc a represen- tation formalism which has been used (under a differ- ent name) as tire basis for the intermediary language in the machine mmshttion system CRITFER (lsahelle et at., 1988; l)ymetman, 1992; Isabelle, 1995). U-forms account for two central aspects of linguistic structure: predicate-argument relations and headedness (comple- ments vs. modifiers), and so form a middle ground be- tween a "semantic" and a "syntactic" representation. This, combined with their formal simplicity, accotmts for much of the popularity of U-forms or related for- malisms -- such as the semantic and deep syntactic representations used in Mel'cuk's Meaning-Text The- ory (Mel'+nk, 1981) -- in applications such as [nit- chine translation and text generation. Although t J-fotn~s are strongly "meaning-oriented" their interpretation is never made explicit but is left to the computational linguist's intu- ition. This has two consequences: • Operations performed on U-forms and related formalisms cannot he controlled for semantic va- lidity. So, for instance, it is comnlon practice to deline graph rewriting rules on these representa- tions which are believed to produce semantically equivalent expressions. Without the check of for+ real interpretation, these rtdes may work in some cases, hut produce wrong results in other cases. So for instance, a rule rewriting (the representa- tion of) "John's salary is $25000 higher this year than last year" into "John's salary was $25000 lower last year than this year" would seem in- tuitively valid until one considered the case of "John's salary is 50% higher this year than last year", where it does not work any rnore. • U-forms are not directly adapted to applications putting emphasis on denotational semantics and formal reasoning, like for instance some lmtural hmguage generation systems in well-formalized domains (l-htang and Fiedler, 1995; Ranta, 1995; I+evine, 1990), see also (Alshawi, 1992). A basic obstacle to providing a formal interpretation for U-forms is the fact that these representations leave the relative scopes of dependents implicit. The S-form representation (Scoped dependency form), which we introduce here, is an extension of U-form notation which makes scope explicit, by allowing dependents to be ordered relative to one another. Dependents (complements or modifiers) c~m move fi'eely relative to one another in the S-form structure, under certain binding-site constraints+ We then go on …

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