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<Paper uid="P95-1013">
  <Title>Compilation of HPSG to TAG*</Title>
  <Section position="6" start_page="98" end_page="98" type="concl">
    <SectionTitle>
4 Conclusion
</SectionTitle>
    <Paragraph position="0"> We have described how HPSG specifications can be compiled into TAG, in a manner that is faithful to both frameworks. This algorithm has been implemented in Lisp and used to compile a significant fragment of a German HPSG. Work is in progress on compiling an English grammar developed at CSLI.</Paragraph>
    <Paragraph position="1"> This compilation strategy illustrates how linguistic theories other than those previously explored within the TAG formalism can be instantiated in TAG, allowing the association of structures with an enlarged domain of locality with lexical items. We have generalized the notion of factoring recursion in TAG, by defining auxiliary trees in a way that is not only adequate for our purposes, but also provides a uniform treatment of extraction from both clausal and non-clausal complements (e.g., VPs) that is not possible in traditional TAG.</Paragraph>
    <Paragraph position="2"> It should be noted that the results of our compilation will not always conform to conventional linguistic assumptions often adopted in TAGs, as exemplified by the auxiliary trees produced for equi verbs. Also, as the algorithm does not currently include any downward expansion from complement nodes on the frontier, the resulting trees will sometimes be more fractioned than if they had been specified directly in a TAG.</Paragraph>
    <Paragraph position="3"> We are currently exploring the possiblity of compiling HPSG into an extension of the TAG formalism, such as D-tree grammars (RVW95) or the UVG-DL formalism (Ram94). These somewhat more powerful formalisms appear to be adequate for some phenomena, such as extraction out of adjuncts (recall SS2) and certain kinds of scrambling, which our current method does not handle. More flexible methods of combining trees with dominance links may also lead to a reduction in the number of trees that must be produced in the second phase of our compilation.</Paragraph>
    <Paragraph position="4"> There are also several techniques that we expect to lead to improved parsing efficiency of the resulting TAG. For instance, it is possible to declare specific non-SFs which can be raised, thereby reducing the number of useless trees produced during the multi-phase compilation. We have also developed a scheme to effectively organize the trees associated with lexical items.</Paragraph>
  </Section>
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