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<Paper uid="P97-1070">
  <Title>Representing Paraphrases Using Synchronous TAGs</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> The context of the paraphrasing in this work is that of Reluctant Paraphrase (Dras, 1997b). In this framework, a paraphrase is a tool for modifying a text to fit a set of constraints like length or lexical density. As such, generally applicable paraphrases are appropriate, so syntactic paraphrases-paraphrases that can be represented in terms of a mapping between syntax trees describing each of the paraphrase alternatives--have been chosen for their general applicability. Three examples are:  (1) a. The salesman made an attempt to wear Steven down.</Paragraph>
    <Paragraph position="1"> b. The salesman attempted to wear Steven down.</Paragraph>
    <Paragraph position="2"> (2) a. The compere who put the contestant to the lie detector gained the cheers of the audience.</Paragraph>
    <Paragraph position="3"> b. The compere put the contestant to the lie detector test. He gained the cheers of the audience.</Paragraph>
    <Paragraph position="4"> (3) a. The smile broke his composure.</Paragraph>
    <Paragraph position="5"> b. His composure was broken by the smile.</Paragraph>
    <Paragraph position="6">  A possible approach for representing paraphrases is that of Chandrasekar et al (1996) in the context of text simplification. This involves a fairly straightforward representation, as the focus is on paraphrases which simplify sentences by breaking them apart. However, for purposes other than sentence simplification, where paraphrases like (1) are used, a more complex representation is needed.</Paragraph>
    <Paragraph position="7"> A paraphrase representation can be thought of as comprising two parts--a representation for each of the source and target texts, and a representation for mapping between them. Tree Adjoining Grammars (TAGs) cover the first part: as a formalism for describing the syntactic aspects of text, they have a number of desirable features. The properties of the formalism are well established (Joshi et al, 1975), and the research has also led to the development of a large standard grammar (XTAG Research Group, 1995), and a parser XTAG (Doran et al, 1994). Mapping between source and target texts is achieved by an extension to the TAG formalism known as Synchronous TAG, introduced by Shieber and Schabes (1990). Synchronous TAGs (STAGs) comprise a pair of trees plus links between nodes of the trees. The original paper of Shieber and Schabes proposed using STAGs to map from a syntactic to a semantic representation, while another paper by Abeill@ (1990) proposed their use in machine translation. The use in machine translation is quite close to the use proposed here, hence the comparison in the following section; instead of mapping between possibly different trees in different languages, there is a mapping between trees in the same language with very different syntactic properties.</Paragraph>
  </Section>
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