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<Paper uid="P01-1019">
  <Title>An Algebra for Semantic Construction in Constraint-based Grammars</Title>
  <Section position="9" start_page="0" end_page="0" type="concl">
    <SectionTitle>
8 Conclusions and future work
</SectionTitle>
    <Paragraph position="0"> We have developed a framework for formally specifying semantics within constraint-based representations which allows semantic operations in a grammar to be tightly specified and which allows a representation of semantic content which is largely independent of the feature structure architecture of the syntactic representation. HPSGs can be written which encode much of the algebra described here as constraints on types in the grammar, thus ensuring that the grammar is consistent with the rules on composition. There are some aspects which cannot be encoded within currently implemented TFS formalisms because they involve negative conditions: for instance, we could not write TFS constraints that absolutely prevent a grammar writer sneaking in a disallowed coindexation by specifying a path into the lzt. There is the option of moving to a more general TFS logic but this would require very considerable research to develop reasonable tractability. Since the constraints need not be checked at runtime, it seems better to regard them as metalevel conditions on the description of the grammar, which can anyway easily be checked by code which converts the TFS into the algebraic representation.</Paragraph>
    <Paragraph position="1"> Because the ERG is large and complex, we have not yet fully completed the exercise of retrospectively implementing the constraints throughout. However, much of the work has been done and the process revealed many bugs in the grammar, which demonstrates the potential for enhanced maintainability. We have modified the grammar to be monotonic, which is important for the chart generator described in Carroll et al (1999). A chart generator must determine lexical entries directly from an input logical form: hence it will only work if all instances of nonmonotonicity can be identified in a grammar-specific preparatory step. We have increased the generator's reliability by making the ERG monotonic and we expect further improvements in practical performance once we take full advantage of the restrictions in the grammar to cut down the search space.</Paragraph>
  </Section>
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