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<Paper uid="C94-1072">
  <Title>and DISCO -- An HPSG-based NLP System its Application for Appointment Scheduling -- Project Note --</Title>
  <Section position="3" start_page="436" end_page="436" type="intro">
    <SectionTitle>
2 Formalism
</SectionTitle>
    <Paragraph position="0"> For the grammar, tile lexicon and parts of tile roof phology a powerfld typed unification lbrmalism 7&amp;quot;/)PS (Type Description Language) has been developed.</Paragraph>
    <Paragraph position="1"> Reasoning is performed by two specialized inference engines, viz. the TD1; type engine and the l~ature constraint-solver llDi.Afe. The modnles are connected via a flexible interface to allow for mutual control. Type System 7&amp;quot;191; is a powerfifl typed featureb~ed language and inference system, spceilicalty suited for highly lexiealized grammars \[81 (in this vohl,ne). Type definitions in &amp;quot;I'19PS consist of type constraints and feature constraints over the standard boolean connectives A, V, and -1. The operators are generalized ill that they can connect feature descril)tions, coreference tags (logical variables) and types. 7&amp;quot;/)1; distinguishes between arm types (open-world remsoning), sort types (closed-world reasoning), bl*ilt-in types, and atoms.</Paragraph>
    <Paragraph position="2"> Recursive types are explicitly allowed and handled by a sophisticated lazy type expavsiot, mechanism.</Paragraph>
    <Paragraph position="3"> 7/;)PS allows the definition of partitions and the declaration of sets of types as incompatible, meaning that the conjunction of them yields L. Working with partially as well ~s with fully exl)anded types is possible through the use of a sophisticated type expansion mechanism, both at definition and at run time. 7-791; is fid\[y incremental in that it allows the redelinition of types and the use of undelined types. 7'/9PS allows a grammarian to define and use parameterized templates (macros). Input given to 7&amp;quot;D1; is parsed by a LAMI.(1) parser to allow for all intuitive, high-lew;l input syntax.</Paragraph>
    <Paragraph position="4"> Eilicient reasoning in the system is accomplished through specialized modules: (i) bit vector encoding of the type subsumption hierarchy; (it) f~ust symbolic simplification for complex type expressions; (iii) memoization to cache precomputed results; and (iv) type expansion to make constraints explicit, to determine the global satisfiability of a description, and to work with partially expanded types during processing.</Paragraph>
    <Paragraph position="5"> Constraint Solver NDiAfe is a feature constraint solver capable of dealing with distributed disiunctions over arbitrary structures, negative coreferences, full negation, and functional and relational constraints.</Paragraph>
    <Paragraph position="6"> It is the first (and to our knowledge t},e only) implemented feature constraint solver that integrates both full negation and distributed disjunctions \[2\]. NDYq'e does l,ot use distributed disjunction only as a tool for efficient processing. It also forms part of the input syntax, which allows for very compact representation of tire input data. In controls( with other systems using distributed disjunctions, we do not restrict disjunctions to length two, thus reducing tire size of the feature structure representations massively.</Paragraph>
    <Paragraph position="7"> ~l'he functionality of N1)iAfe is completed by several auxiliary functimls. It is possible to remove inconsistent alternatives, to si,nplify structures, to extract subterms or to evaluate functional constraints. One call also construct disjunctive normal form if desired.</Paragraph>
    <Paragraph position="8"> Semantic Representation A specialized meaning representation formalism, Aft;t, developed at llewlctt Packard \[9\], is used for semantic reasoning and as a flexible interface to various application systems. HPS1; is a linguistically motivated extension of sorted \[irstorder predicate logic, integrating also eonccl)ts from Situation Semantics and l)l{'|'. It provides a large range of rei)resentational mechanisms for natural language phenomena.</Paragraph>
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