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<Paper uid="C90-3033">
  <Title>When Something Is Missing: Ellipsis, Coordination and the Chart</Title>
  <Section position="1" start_page="0" end_page="0" type="abstr">
    <SectionTitle>
Abstract
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    <Paragraph position="0"> This paper deals with two linguistic phenomena which are usually considered cases of ill-formedness by the computational linguistics community: intersentential ellipsis and coordination (possibly with gaps). We present an original solution, if compared to those already known for the two phenomena.</Paragraph>
    <Paragraph position="1"> This solution is conceived within a relevant approach to parsing, i.e. chart parsing, and is coherent with the basic ideas of this approach.</Paragraph>
    <Paragraph position="2"> I. Introduction The ability to face and resolve problems associated with ill-formedness is fundamental in order to make natural language interfaces usable (see \[Carbonell and Hayes, 1983\] for a review of the problems of ill-formedness and of techniques used to resolve them). We shall focus on two phenomena: intersentential ellipsis and coordination (possibly with gaps).</Paragraph>
    <Paragraph position="3"> Ellipsis is a very common phenomenon and is frequently encountered in dialogues between persons. Up to the present, studies on natural language interaction with computers generally highlight the frequency of this phenomenon, (see, for example, \[Eastman and McLean, 1981\]).</Paragraph>
    <Paragraph position="4"> For this reason, ellipsis has received much attention and different solutions have been proposed based on the mechanism used to analyze the sentence: semantic grammars (the LIFER/LADDER system \[Hendrix, 1977\]), ATN \[Kwasny and Sondheimer, 1981, Weischedel and Sondheimer, 1982\], or case-frame instantiation (the XCALIBUR system \[Carbonell et al., 1983\]).</Paragraph>
    <Paragraph position="5"> As far as coordination is concerned, it is also frequently considered a phenomenon of ill-fomaedness for the following reasons: - since every pair of constituents of the same syntactic category may be coordinated, if the grammar specifically included all these possibilities it would greatly increase the size of the grammar itself; - a constituent inside a coordination may have gaps (that is, missing elements) that, in general, are not allowed in constituents of the same type.</Paragraph>
    <Paragraph position="6"> Even in the most purely linguistic area, coordination has not received in-depth treatment, if not recently (for example, see \[Sag et al., 1984\] and \[Kaplan and Maxwell, 1988\]).</Paragraph>
    <Paragraph position="7"> Until the beginning of the 80's almost no system (the most relevant exception being the SYSCONJ module, present in the LUNAR system \[Woods, 1973\]) has confronted coordination in a generalized manner. The 80's have seen renewed computational interest in coordination that has brought new efforts (see \[Kwasny and Sondheimer, 1981\], \[Dahl and McCord, 1983\], \[Fong and Berwick, 1985\], \[Lesmo and Torasso, 1985\], \[Kosy, 1986\] and \[Proudian and Goddeau, 1987\]).</Paragraph>
    <Paragraph position="8"> In this paper we present a solution to both the problems outlined above (that is, ellipsis and coordination). The solution is original with respect to those presented in the literature in that it is based on chart parsing \[Kaplan, 1973, Kay, 1980\], an approach little used to treat iU-formedness until now. t For both problems the solution is based on a strategy that uses the information contained in the structures produced by the parser (as is the case in almost all the work mentioned in this introduction; for a pragmatics-based approach to ellipsis, see \[Carberry, 1989\]).</Paragraph>
    <Paragraph position="9"> Both the solutions proposed have been inserted into WEDNESDAY 2 \[Stock, 1989\], a chart-based parser used in a dialogue system for Italian; attention has been paid to distinctive aspects of this language (for example, the relative liberty of order inside a single constituent). The process of building the analysis is based on a sort of unification. The parser is built so that it may be connected to a semantic (and possibly pragmatic) discrimination component; in this way, the discrimination is carried out incrementally as the syntactic analysis proceeds. All examples in this paper are in Italian.</Paragraph>
  </Section>
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