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<Paper uid="J88-1001">
  <Title>CATEGORY STRUCTURES</Title>
  <Section position="15" start_page="11" end_page="11" type="evalu">
    <SectionTitle>
7 CONCLUSION
</SectionTitle>
    <Paragraph position="0"> We have developed and applied a general framework for defining syntactic categories, including categories in which features can have categories as their value, which latter possibility turns out to subsume the possibility of a feature taking as its value a list of indices or categories, drawn from either a finite or an infinite set. The unitary way in which we have characterized these diverse systems is intended to assist in the exploration and comparison of grammatical formalisms. Questions concerning whether particular rule types and operations on categories that are familiar from one approach to grammar can be carried over unproblematically to another approach, and questions concerning the implementation difficulties that arise when a given formalism is adopted, can in many cases be settled in a straight-forward and familiar way, namely by reducing them to previously encountered types of question.</Paragraph>
    <Paragraph position="1"> The grammatical frameworks we have considered as examples fall into a five-class typology which we can now explicate. The first class contains the frameworks that use only atom-valued features (simple phrase structure grammar, Harman's augmented phrase structure grammar; RG and APG); the second contains the special case of GKPS, which uses category-valued features but imposes a constraint which prevents them from having effects on expressive power that could not ultimately by simulated by atom-valued features; the third contains the frameworks that use just a single category-valued feature (our key example being indexed grammar); the fourth contains frameworks making use of more than one category-valued feature (an example being categorial grammar); and the fifth includes those frameworks that fall outside the scheme we have developed in that their categories are not representable as finite partial functions constrained by statements in L c (LFG, FUG, PATR II, etc.).</Paragraph>
    <Paragraph position="2"> It is not at all clear which of these five classes of approaches will prove the most suitable for implementing natural language processing systems in the long term. In this paper, we hope to have made somewhat clearer the nature of the issues at stake. We hope also to have done something more: for the first four classes, we have provided what is in effect a unitary type of data structure for the representation of their syntactic categories. Thinking in terms of such data structures should make it possible for pseudo-issues in natural language processing research to be avoided in a large class of circumstances, to the point that even a decision in mid-project to change the grammatical framework from one linguistic approach to another need not entail any fundamental redesign of what are in most frameworks the basic objects of syntactic representation.</Paragraph>
  </Section>
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