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<Paper uid="P98-1061">
  <Title>A structure-sharing parser for lexicalized grammars</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> It is well-known that fully lexicalised grammar formalisms such as LTAG (Joshi and Schabes, 1991) are difficult to parse with efficiently. Each word in the parser's input string introduces an elementary tree into the parse table for each of its possible readings, and there is often a substantial overlap in structure between these trees. A conventional parsing algorithm (Vijay-Shanker and Joshi, 1985) views the trees as independent, and so is likely to duplicate the processing of this common structure. Parsing could be made more efficient (empirically if not formally), if the shared structure could be identified and processed only once.</Paragraph>
    <Paragraph position="1"> Recent work by Evans and Weir (1997) and Chen and Vijay-Shanker (1997) addresses this problem from two different perspectives. Evans and Weir (1997) outline a technique for compiling LTAG grammars into automata which are then merged to introduce some sharing of structure. Chen and Vijay-Shanker (1997) use underspecified tree descriptions to represent sets of trees during parsing. The present paper takes the former approach, but extends our previous work by: * showing how merged automata can be minimised, so that they share as much structure as possible; * showing that by precompiling additional information, parsing can be broken down into recognition followed by parse recovery; * providing a formal treatment of the algorithms for transforming and minimising the grammar, recognition and parse recovery.</Paragraph>
    <Paragraph position="2"> In the following sections we outline the basic approach, and describe informally our improvements to the previous account. We then give a formal account of the optimisation process and a possible parsing algorithm that makes use of it 1 .</Paragraph>
  </Section>
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