Encoding Lexicalized Tree Adjoining Grammars with a Nonmonotonic Inheritance Hierachy

This paper shows how DATR, a widely used formal language for lexical knowledge representation , can be used to define an I_TAG lexicon as an inheritance hierarchy with internal lexical rules. A bottom-up featu-ral encoding is used for LTAG trees and this allows lexical rules to be implemented as covariation constraints within feature structures. Such an approach eliminates 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 subsequently 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 Research Group, 1995). The advent of such large grammars gives rise to questions of efficient representation , 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 suggestion by showing how the lexical knowledge representation language (LKRL) DA'lR (Evans & Gazdar, 1989a; Evans & Gazdar, 1989b) can be used to formulate 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 inherent in [-TAG lexicons that are expressed in a flat manner with no sharing of structure or properties across the elementary trees. For example, XTAG currently 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 related 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. …