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<Paper uid="P92-1022">
  <Title>An Alternative Conception of Tree-Adjoining Derivation*</Title>
  <Section position="7" start_page="178" end_page="178" type="evalu">
    <SectionTitle>
6 Recognition and Parsing
</SectionTitle>
    <Paragraph position="0"> Following Schabes (1991), the LIG generated by compiling a TAG can be used as the basis for Ear-Icy recognition. Schabes's original method must be modified to respect the differences in compilation engendered by extended derivations. Such parsing rules, along with an extension that allows building of explicit derivation trees on-line as a basis for incremental interpretation, have been developed, and are presented in an extended version of this paper (Schabes and Shieber, 1992). In summary, the algorithm operates as a variant of Earley parsing on the corresponding LIG. The set of extended derivations can subsequently be recovered from the set of Earley items generated by the algorithm. The resultant algorithm can be further modified so as to build an explicit derivation tree incrementally as parsing proceeds; this modification, which is a novel result in its own right, allows the parsing algorithm to be used by systems that require incremental processing with respect to tree-adjoining grammars.</Paragraph>
    <Paragraph position="1"> As a proof of concept, the parsing algorithm just described was implemented in Prolog on top of a simple, general-purpose, agenda-based inference engine. Encodings of explicit inference rules are essentially interpreted by the inference engine.</Paragraph>
    <Paragraph position="2"> The Prolog database is used as the chart; items not already subsumed by a previously generated item are asserted to the database as the parser runs. An agenda is maintained of potential new items. Items are added to the agenda as inference rules are triggered by items added to the chart.</Paragraph>
    <Paragraph position="3"> Because the inference rules are stated explicitly, the relation between the abstract inference rules described in this paper and the implementation is extremely transparent. Because the prototype was implemented as a meta-interpreter it is not particularly efficient. (In particular, the implementation does not achieve the theoretical O(n 6) bound on complexity, because of a lack of appropriate indexing.) Code for the prototype implementation is available for distribution electronically from the authors.</Paragraph>
  </Section>
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