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<Paper uid="P95-1011">
  <Title>Encoding Lexicalized Tree Adjoining Grammars with a Nonmonotonic Inheritance Hierarchy</Title>
  <Section position="3" start_page="0" end_page="77" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> 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 &amp; Schabes (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 (Doran 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 &amp; Gazdar(1992)). In this paper we explore this suggestion by showing how the lexical knowledge representation language (LKRL) DA'lR (Evans &amp; Gazdar, 1989a; Evans &amp; Gazdar, 1989b) can be used to formulate a compact, hierarchical encoding of an \[-'lAG.</Paragraph>
    <Paragraph position="1"> The issue of efficient representation for I_'rAG 1 is discussed by Vijay-Shanker &amp; Schabes (1992), who  I_TAG between the lexicon and the grammar: tile gramrnatical rules are just lexical properties.</Paragraph>
    <Paragraph position="2"> 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.</Paragraph>
    <Paragraph position="3"> 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.</Paragraph>
    <Paragraph position="4"> Vijay-Shanker &amp; Schabes address this problem by introducing a hierarchical lexicon structure with monotonic inheritance and lexical rules, using an approach loosely based on that of Flickinger (1987) but tailored for \[TAG trees rather than HPSG sub-categorization lists. Becker (1993; 1994) proposes a slightly different solution, combining an inheritance component and a set of metarules 2. We share their perception of the problem and agree that adopting a hierarchical approach provides the best available solution to it. However, rather than creating a hierarchical lexical formalism that is specific to the \[_TAG problem, we have used DATR, an LKR.L that is already quite widely known and used. From an \[TAG perspective, it makes sense to use an already available LKRL that was specifically designed to address these kinds of representational issues. From a DATR perspective, I_TAG presents interesting problems arising from its radically lexicalist character: all grammatical relations, including unbounded dependency constructions, are represented lexically and are thus open to lexical generalization.</Paragraph>
    <Paragraph position="5"> There are also several further benefits to be gained from using an established general purpose LKRL such as DATR. First, it makes it easier to compare the resulting \[TAG lexicon with those associated with other types oflexical syntax: there are existing DATR  lexicon fragments for HPSG, PATR and Word Grammar, among others. Second, DATR is not restricted to syntactic description, so one can take advantage of existing analyses of other levels of lexical description, such as phonology, prosody, morphology, compositional semantics and lexical semantics 3. Third, one can exploit existing formal and implementation work on the language 4.</Paragraph>
  </Section>
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