Encoding Lexicalized Tree Adjoining Grammars with a Nonmonotonic Inheritance Hierarchy

Encoding Lexicalized Tree Adjoining Grammars with a Nonmonotonic Inheritance Hierarchy Roger Evans Information Technology Research Institute University of Brighton rpe©itri, bton. ac. uk Gerald Gazdar School of Cognitive Computing Sciences University of Sussex geraldg©cogs, susx. ac. uk David Weir School of Cognitive ~z Computing Sciences University of Sussex dav±dw©cogs, susx. ac. uk Abstract This paper shows how DATR, a widely used formal language for lexical knowledge re- presentation, can be used to define an I_TAG lexicon as an inheritance hierarchy with in- ternal lexical rules. A bottom-up featu- ral encoding is used for LTAG trees and this allows lexical rules to be implemen- ted as covariation constraints within fea- ture structures. Such an approach elimina- tes the considerable redundancy otherwise associated with an LTAG lexicon. 1 Introduction The Tree Adjoining Grammar (lAG) formalism was first introduced two decades ago (3oshi et al., 1975), and since then there has been a steady stream of theoretical work using the formalism. But it is only more recently that grammars of non-trivial size have been developed: Abeille, Bishop, Cote & Scha- bes (1990) describe a feature-based Lexicalized Tree Adjoining Grammar ([_'lAG) for English which sub- sequently became the basis for the grammar used in the XTAG system, a wide-coverage [_TAG parser (Do- ran et al., 1994b; Doran et al., 1994a; XTAG Rese- arch Group, 1995). The advent of such large gram- mars gives rise to questions of efficient representa- tion, and the fully lexicalized character of the [TAG formalism suggests that recent research into lexical representation might be a place to look for answers (see for example Briscoe ef a/.(1993); Daelemans & Gazdar(1992)). In this paper we explore this sugge- stion by showing how the lexical knowledge repre- sentation language (LKRL) DA'lR (Evans & Gazdar, 1989a; Evans & Gazdar, 1989b) can be used to for- mulate a compact, hierarchical encoding of an [-'lAG. The issue of efficient representation for I_'rAG 1 is discussed by Vijay-Shanker & Schabes (1992), who 1As with all fully lexicMized grammar formalisms, there is really no conceptual distinction to be drawn in I_TAG between the lexicon and the grammar: tile gram- rnatical rules are just lexical properties. draw attention to the considerable redundancy in- herent in [-TAG lexicons that are expressed in a flat manner with no sharing of structure or properties across the elementary trees. For example, XTAG cur- rently includes over 100,000 lexemes, each of which is associated with a family of trees (typically around 20) drawn from a set of over 500 elementary trees. Many of these trees have structure in common, many of the lexemes have the same tree families, and many of the trees within families are systematically rela- ted in ways which other formalisms capture using transformations or metarules. However, the [TAG formalism itself does not provide any direct support for capturing such regularities. Vijay-Shanker & Schabes address this problem by introducing a hierarchical lexicon structure with mo- notonic inheritance and lexical rules, using an ap- proach loosely based on that of Flickinger (1987) …

Content truncated to 500 words