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<Paper uid="A88-1032">
  <Title>Localizing Expression of Ambiguity</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Ambiguity is a problem in any natural language processing system. Large grammars tend to produce large numbers of alternative analyses for even relatively simple sentences.</Paragraph>
    <Paragraph position="1"> Furthermore, as is well known, syntactic information may be insu~cient for selecting a best reading. It may take semantic knowledge of arbitrary complexity to decide which alternative to choose.</Paragraph>
    <Paragraph position="2"> In the TACITUS project \[Hobbs, 1986; Hobbs and Martin, 1987\] we are developing a pragmatics component which, given the logical form of a sentence, uses world knowledge to solve various interpretation problems, the r~=oluti,JD of syntactic ambiguity among them. Sentences are translated into logical form by the DIALOGIC system for syntactic mid semantic analysis \[Grosz et al., 1982\]. In this paper we describe how information about alternative parses is passed concisely from DIALOGIC to the pragmatics component, and more generally, we discuss a method of localizing the representation of syntactic ambiguity in the logical form of a sentence.</Paragraph>
    <Paragraph position="3"> One possible approach to the ambiguity problem would be to produce a set of logical forms for a sentence, one for each parse tree, and to send them one at a time to the pragrnatics component. This involves considerable duplication of effort if the logical forms are largely the same and differ only with respect to attachment. A more efficient approach is to try to localize the information about the alternate possibilities.</Paragraph>
    <Paragraph position="4"> Instead of feeding two logical forms, which differ only with respect to an attachment site, to a pragraatics component, it is worthwhile trying to condense the information of the two logical forms together into one expression with a disjunction inside it representing the attachment ambiguity. That one expression may then be given to a pragmatics component with the effect that parts of the sentence that would have been processed twice are now processed only once. The savings can be considerably more dramatic when a set of five or ten or twenty logical forms can be reduced to one, as is often the case.</Paragraph>
    <Paragraph position="5"> In effect, this approach translates the syntactic ambiguity problem into a highly constrained coreference problem. It is as though we translated the sentence in (1) into the two sentences in (2)  (1) John drove down the street in a car.</Paragraph>
    <Paragraph position="6"> (2) John drove down the street. It was in a car.</Paragraph>
    <Paragraph position="7">  where we knew &amp;quot;it&amp;quot; had to refer either to the street or to the driving. Since coreference is one of the phenomena the pragmatics component is designed to cope with \[Hobbs and Martin, 1987\], such a translation represents progress toward a solution.</Paragraph>
    <Paragraph position="8"> The rest of this paper describes the procedures we use to produce a reduced set of logical forms from a larger set. The basic strategy hinges on the idea of a neutral representation \[Hobbs, 1982\]. This is similar to the idea behind Church's Pseudo-attachment \[Church, 19801 .</Paragraph>
    <Paragraph position="9"> Pereira's Rightmost Normal Form \[Pereira, 1983\], and what Rich et al. refer to as the Procr~tination Approach to parsing \[Rich, Barnett, Wittenburg, and Whittemore.</Paragraph>
    <Paragraph position="10"> 1986\]. However, by expressing the ambiguity as a disjunction in logical form, we put it into the form most convenient for subsequent inferential processing.</Paragraph>
  </Section>
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