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<Paper uid="P86-1013">
  <Title>PARSING CONJUNCTIONS DETERMINISTICALLY</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> In recent years, there has been an upsurge of interest in techniques for parsing sentences containing coordinate conjunctions (and, or and but) \[1,2,3,4,5,8,9\]. These techniques are intended to deal with three computational problems inherent in conjunc- null tion parsing: 1. Since virtually any pair of constituents of the same syntactic type may be conjoined, a grammar that explicitly enumerates all the possibilities seems needlessly cluttered with a large number of conjunction rules.</Paragraph>
    <Paragraph position="1"> 2. If a parser uses a top-down analysis strategy (as is  common with ATN and logic grammars), it must hypothesize a structure for the second conjunct without knowledge of its actual structure. Since this structure could be any that parallels some constituent that ends at the conjunction, the parser must generate and test all such possibilities in order to find the ones that match. In practice, the combinatorial explosion of possibilities makes this slow.</Paragraph>
    <Paragraph position="2"> 3. It is possible for a conjunct to have &amp;quot;gaps&amp;quot; (ellipsed elements) which are not allowed in an unconjoined constituent of the same type. These gaps must be filled with elements from the other conjunct for a proper interpretation, as in: I gave Mary a nickel and Harry a dime.</Paragraph>
    <Paragraph position="3"> The paper by Lesmo and Torasso \[9\] briefly reviews which tech. niques apply to which problems before presenting their own approach. null Two papers in the list above \[1,3\] present deterministic, &amp;quot;wait. and-see&amp;quot; methods for conjunction parsing. In both, however, the discussion centers around the theory and feasibility of parsers that obey the Marcus determinism hypothesis \[10\] and operate with a limited-length Iookahead buffer. This paper examines the other side of the coin, namely, the practical power of the waitand.see approach compared to strictly top-down or bottom-up methods. A parser is described that analyzes conjunction struc. tures deterministically and produces parse trees similar to those produced by Dahl &amp; McCord's MSG system \[4\]. It is much faster than either MSG or Fong &amp; Berwick's RPM device \[5\], and comparative timings are given. We conclude with some descriptive comparisons to other systems and a discussion of the reasons behind the performance observed.</Paragraph>
  </Section>
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