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<Paper uid="E93-1008">
  <Title>Disjunctions and Inheritance in the Context Feature Structure System</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> The Context Feature Structure System (CFS) \[BSttcher and KSnyves-Tdth, 1992\] is a unification based system which evaluates feature structures with distributed disjunctions and dynamically definable types for structure inheritance. CFS is currently used to develop and to test a dependency grammar for German in the text analysis project KONTEXT.</Paragraph>
    <Paragraph position="1"> In this paper disjunctions and inheritance will be investigated with regard to both, their application dimension and their efficient computational treatment.</Paragraph>
    <Paragraph position="2"> The unification algorithm of CFS and the concept of virtual agreements for structure sharing has been introduced in \[BSttcher and KSnyves-TSth, 1992\]. The algorithm handles structure inheritance by structure sharing and constraint sharing which avoids copying of path structures and constraints completely. Disjunctions are evaluated concurrently without backtracking and without combinatoric multiplication of the path structure. For that purpose the path structure is separated from the structure of disjunctions by the introduction of contexts.</Paragraph>
    <Paragraph position="3"> Contexts are one of the key concepts for maintaining disjunctions in feature terms. They describe readings of disjunctive feature structures. We define them slightly different from the definitions in \[DSrre and Eisele, 1990\] and \[Backofen et ai., 1991\], with a technical granularity which is more appropriate for their efficient treatment. The CFS unification algorithm computes a set of nogood contexts for all conflicts which occur during unification of structures.</Paragraph>
    <Paragraph position="4"> An algorithm for contexts which computes from a set of nogoods whether a structure is valid, will be described in this paper. It is a specialized search procedure which avoids the investigation of the full search space of contexts by clustering disjunctions.</Paragraph>
    <Paragraph position="5"> We start with some examples how disjunctions and inheritance are used in the CFS environment. Then contexts are formally defined on the basis of the semantics of CFS feature structures. Finally the algorithm computing validity of contexts is outlined.</Paragraph>
  </Section>
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