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<Paper uid="P95-1012">
  <Title>Compiling HPSG type constraints into definite clause programs</Title>
  <Section position="3" start_page="85" end_page="85" type="intro">
    <SectionTitle>
2 Background
2.1 The HPSGII architecture
</SectionTitle>
    <Paragraph position="0"> A HPSG grammar consists of two components: the declaration of the structure of the domain of linguistic objects in a signature (consisting of the type hierarchy and the appropriateness conditions) and the formulation of constraints on that domain. The signature introduces the structures the linguist wants to talk about. The theory the linguist proposes distinguishes between those objects in a domain which are part of the natural language described, and those which are not.</Paragraph>
    <Paragraph position="1"> HPSGII gives a closed world interpretation to the type hierarchy: every object is of exactly one minimal (most specific) type. This implies that every object in the denotation of a non-minimal type is also described by at least one of its subtypes. Our compilation procedure will adhere to this interpretation. null</Paragraph>
    <Section position="1" start_page="85" end_page="85" type="sub_section">
      <SectionTitle>
2.2 The theories of HPSGII: Directly
</SectionTitle>
      <Paragraph position="0"> constraining the domain A HPSGII theory consists of a set of descriptions which are interpreted as being true or false of an object in the domain. An object is admissible with respect to a certain theory iff it satisfies each of the descriptions in the theory and so does each of its substructures. The descriptions which make up the theory are also called constraints, since these descriptions constrain the set of objects which are admissible with respect to the theory.</Paragraph>
      <Paragraph position="1"> Figure 1 shows an example of a constraint, the head-feature principle of HPSGII. Throughout the paper we will be using HPSG style AVM notation for descriptions.</Paragraph>
      <Paragraph position="2">  The intended interpretation of this constraint is that every object which is being described by type phrase and by \[DTI~S h~aded-str~c\] also has to be described by the consequent, i.e. have its head value shared with that of its head-daughter.</Paragraph>
      <Paragraph position="3"> In the HPSG II architecture any description can be used as antecedent of an implicative constraint. As shown in (Meurers, 1994), a complex description can be expressed as a type by modifying the signature and/or adding theory statements. In the following, we therefore only deal with implicative constraints with type antecedents, the type definitions.</Paragraph>
    </Section>
    <Section position="2" start_page="85" end_page="85" type="sub_section">
      <SectionTitle>
2.3 Theories in constraint logic
</SectionTitle>
      <Paragraph position="0"> programming: expressing definite clause relations As mentioned in the introduction, in most computational systems for the implementation of HPSG theories a grammar is expressed using a relational extension of the description language 2 such as definite clauses or phrase structure rules. Figure 2 schematically shows the embedding of HPSG II descriptions in the definition of a relation.</Paragraph>
      <Paragraph position="1">  relo (D1 ..... D~) :- tell(E1,..., Ej), re/n(Fl .... , Fh).</Paragraph>
      <Paragraph position="2">  The HPSG description language is only used to specify the arguments of the relations, in the example noted as D, E, and F. The organization of the descriptions, i.e. their use as constraints to narrow down the set of described objects, is taken over by the relational level. This way of organizing descriptions in definite clauses allows efficient processing techniques of logic programming to be used.</Paragraph>
      <Paragraph position="3"> The question we are concerned with in the following is how a HPSG II theory can be modelled in such a setup.</Paragraph>
    </Section>
  </Section>
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