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<Paper uid="P93-1028">
  <Title>A LOGICAL SEMANTICS FOR NONMONOTONIC SORTS</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> There have been many suggestions for incorporating defaults into unification-based grammar formalisms (Bouma 1990; Bouma 1992; Carpenter 1991; Kaplan 1987; Russell et al. 1992; Shieber 1986; Shieber 1987). Each of these proposes a non-commutative, non-associative default unification operation that combines one structure representing strict information with another representing default information. When presented with a set of structures, the result depends on the order in which the structures are combined. This runs very much against the unification tradition, in which any set has a unique most general satisfier (if a satisfier exists at all).</Paragraph>
    <Paragraph position="1"> A method that is free of these ordering effects was presented in (Young 1992). The method of nonmonotonic sorts (NSs) allows default labels to be assigned at any time, and used only in the absence of conflicting information. NSs replace the more traditional labels on feature structures to give nonmonotonically sorted feature structures (NS-FSs). These structures can be combined by an associative and commutative unification operation. FSs are rederived from NSFSs by taking a solution--an operation defined in terms of information present in the NSFS.</Paragraph>
  </Section>
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