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<Paper uid="P88-1029">
  <Title>Conditional Descriptions in Functional Unification Grammar</Title>
  <Section position="6" start_page="238" end_page="238" type="concl">
    <SectionTitle>
5 Summary
</SectionTitle>
    <Paragraph position="0"> We have shown how the notational resources of FUG can be extended to include descriptions of conditional information about feature structures. Conditional descriptions have been given a precise logical definition in terms of the feature description logic of Kasper and Rounds, and we have shown how a unification method for feature descriptions can be extended to use conditional descriptions. We have implemented this unification method and tested it in a parser for systemic grammars, using several hundred conditional descriptions.</Paragraph>
    <Paragraph position="1"> The definition of conditional descriptions and the unification method should be generaily applicable as an extension to other unificatlon-based grammar frameworks, as well as to FUG and the modeling of systemic grammars. In fact, the implementation described has been carried out by extending PATI~II \[Shie84\], a general representational framework for unificatlon-based grammars.</Paragraph>
    <Paragraph position="2"> While it is theoretically possible to represent the information of conditional descriptions indirectly using notational devices already present in Kay's FUG, there are practical advantages to representing conditional descriptions directly.</Paragraph>
    <Paragraph position="3"> The indirect encoding of conditional descriptions by dlsjunctions and negations entails approximately doubling the size of a description, adding many explicit nonexistence constraints on features (NONE values), and slowing the unification process. In our experiments, unification wlth conditional descriptions requires approximately 50~ of the time required by unification with an indirect encoding of the same descriptions. By adding conditional descriptions as a notational resource to FUG, we have not changed the theoretical limits of what FUG can do, but we have developed a representation that is more perspicuous, less verbose, and computationaily more e/~clent.</Paragraph>
  </Section>
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