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<Paper uid="C92-1034">
  <Title>Structure Sharing in Lexicalized Tree-Adjoining Graulmars*</Title>
  <Section position="5" start_page="0" end_page="0" type="metho">
    <SectionTitle>
4 Lexical and Syntactic Rules
</SectionTitle>
    <Paragraph position="0"> The second mechanism we adopt for structure-sharing is the use of lcxical and syntactic rules to capture inflectional and derivational relationships among lexical entries. The mechanism is very similar than the one proposed by Flickinger (1987), however it ditfers from it since we derive elementary trees of extended domain of locality. Lexical and syntactic rules relate an input lexical entry to an output lexical entry. The output lexical entry gets its information from the input lexical entry, lexical and syntactic rules, and possibly additional idiosyncratic information specified in the lexicon.</Paragraph>
    <Paragraph position="1"> We illustrate the use of lexical and syntactic rules by examples. In the following, we will focus our attention to the derivational relationships and also to the output tree description. Consider the rule for wh-question.</Paragraph>
    <Paragraph position="3"> We treat formation of structure for wb-question as relation between two lexical entries specified here as LEI and LEo. The tree description ill LEo indicates that an argument node (x) ill tile tree described in LEi can be moved leaving a trace. Ilere tile relationship between x and y is obtained from the description in LEI. Copy(x) indicates that a copy of the description of entire sub-tree rooted at node x needs to recorded in output description. In the resulting description, the filler is shown to C-command the gap.</Paragraph>
    <Paragraph position="4"> ACRES DE COL1NG-92, NANTES, 23-28 AOI~'I' 1992 2 0 9 Plloc. OF C()I.IN(;-92, NAI, rrI';S, Auo. 23-28, 1992 Thus, if LEI stood for DITRANSI and say we consider x to be the node NP direct object, the trees described in LEi and LEo are:  p np e P NIP Before illustrating the passive rule, we need to introduce the so-called CHANGE ARITY relation introduced by Fiickinger (1987). We say that C~ = CItANGE-ARITY(C1) if CI is the immediate superclass of C1 distinct from TRANSITIVE. We can now state the passive rule:  Suppose we let LEi.class to be DITRANS1. Thus from the definition CHANGE-ARITY(DITRANS1) is PP-IOBJ. The tree description inherited from PP.</Paragraph>
    <Paragraph position="5"> IOBJ differs from that of DITRANS1 only in that we do not postulate the presence of the node for NP direct object. Thus the tree description we arrive at is:  From the tree description and constraint equations of the passive class, we will inherit information to place the two feature structures on the VP nodes on top form:passive and on the bottomfevm:pparl. Since these feature structures cannot be unified, the auxiliary verb &amp;quot;be&amp;quot; is required to adjoin on the VP node. As in any similar enterprise, we have to provide means to handle exceptions. For example, we will have to provide such mechanisms to handle verbs that are exceptions to the use of PASSIVE or DATIVE rule.</Paragraph>
    <Paragraph position="6"> Like in (Flickinger, 1987) we allow overwriting and state explicitly that certain rules are not applicable in the entries of some lexieal items. However, considerable more machinery would need to be added to capture semantic constraints on the application of such rules. At present, little work has been done to incorporate specification of semantic constraints in conjunction with TAG.</Paragraph>
  </Section>
  <Section position="6" start_page="0" end_page="0" type="metho">
    <SectionTitle>
5 Conclusion
</SectionTitle>
    <Paragraph position="0"> While a number of proposals (Flickinger, 1987; Pollard and Sag, 1987; Shieber, 1986) have been made for a hierarchical organization of a lexicon and grammar, in our approach the hierarchical lexicon, the syntactic and lexical rules additionally specify partial descriptions of trees of extended domain of locality which capture syntactic mid semantic dependencies. The description of elementary trees has been obtained by collecting par-Acrv.s DE COLING-92, NAMES, 23-28 AOt~n&amp;quot; 1992 2 1 0 PROC. OF COLING-92, NANTES, AUG. 23-28, 1992 tim description of trees and then realizing tile least tree satisfying these constraints. Tile syntactic and lexical rules enable us to derive new entries from existing ones.</Paragraph>
    <Paragraph position="1"> Overwriting allows us to be sensitive to lexical idiosyner asies.</Paragraph>
    <Paragraph position="2"> As mentioned earlier, tile linguistic exaraples given here were meant only to indicate the potential of our approach. In general, we anvisage tile use of a hierarchical lexicon, of syntactic and lexical rules for lexicalized tree-adjoining grammars that capture importaalt linguistic generalizations and provide for a space efficient representation of tile grammar. Equany important, such a scheme would facilitate the automation of the process of updating and maintaining the grammar, an urgent need felt during the development of a large lexica\[ized tree-adjoining grammar.</Paragraph>
    <Paragraph position="3"> We are currently investigating the possibility of defining parsing strategies that take advantage of the type of hierarchical representation we proposed ill this paper. Many other related topics will be explored ill the future. A much more elaborate organization will be considered, which in turn may suggest the need for additional machinery. We will implement the inheritance machinery described above and the process of building trees from these descriptions. We would also like to consider the treatment of idioms and tile integration of syntactic and semantic specifications in the context of LTAG.</Paragraph>
  </Section>
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