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<Paper uid="E91-1029">
  <Title>Composite Atomic Values</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
PHON
SYN\[LOC HEADIMAJ SUBCAT
</SectionTitle>
    <Paragraph position="0"> * an NP such as a complete snob, having description null PHOt~ a complete snob \] s..cA, &lt;)jj * or ~a AP ~ NP such as famous and a complete snob, having description s sPHON famous and a complete snob &amp;quot; The subcategorlsation requirements may not, however, by satisfied by, for example, a PP, or any conjunctive composite containing a PP. Hence the examples in (3) are not Mmitted.</Paragraph>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
Implementation Issues
</SectionTitle>
      <Paragraph position="0"> The problems of implementing a system involving composites really stem fromtheir requirement for a proper implementation of disjunction. Implementation may be approached by adopting a strict division between the objects of the language and the objects of the described world. According to this approach, and in Prolog, Prolog terms ~re taken to correspond to the objects in the semantic domain, with Prolog clauses being interpreted much as in the syntax of an equational logic, as constraints on those terms. Conjunctive constraints correspond to unification. The formation of conjunctive composites is also no problem: such objects exist in the semantic domain, so structured terms may be constructed whose subterms are the elements of the composite. Thus if we implement the composite connectives as binary operators, * for ~ and + for ~, we may form Prolog terms (A * B) corresponding to conjunctive composites. Disjunction, and the use of disjunctive composites, cannot, however, be implemented in the same way. The problem with disjunction is that we cannot normally be sure which disjunct is appropriate, and a term of the form (A + B) will not unify with the term A, as is required by either form of disjunction. The freeze/2 predicate of many second generation Prologs provides some help here. For standard aWe assume that the rule licensing coordinate structures unifies all corresponding values (such as the vahies for each SUBCAT attribute) except for the values of the HEAD attributes. The value of the HEAD attribute of the coordinate structure is the composite of the values of the HEAD attribute of each conjunct.</Paragraph>
      <Paragraph position="1">  disjunction, we might augment feature structure unification clauses (using &lt;=&gt; to represent the unification operator and \/to represent standard disjunction) with special clauses such as:  The idea is that the ~reeze/2 predicate delays the evaluation of disjunctive constraints until the relevant structure is sufficiently instantiated. Unfortunately, &amp;quot;sufficiently instantiated&amp;quot; here means that it is nonvar. Only in the case of atoms is this normally sufficient. Thus the above approach is suitable for the implementation of composites at the level of atoms, but not suitable in the wider domain of composite feature structures.</Paragraph>
      <Paragraph position="2"> Concluding Remarks In giving a treatment of coordination, and in particular cross-categorial coordination, within a unification-based grammar formalism we have introduced composite feature structures which describe composite objects. A sharp distinction is drawn between syntax and semantics: in the semantic domain there is only one variety of composite object, but in the syntactic domain there are two forms of composite description, a conjunctive composite description and a disjunctive composite description. Satisfiability conditions are given for the connectives in terms of a generalised notion of deterministic finite state automaton. Some issues which arise in the Prolog implementation of the connectives are also discussed.</Paragraph>
    </Section>
  </Section>
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