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<Paper uid="P92-1026">
  <Title>HANDLING LINEAR PRECEDENCE CONSTRAINTS BY UNIFICATION</Title>
  <Section position="2" start_page="0" end_page="201" type="intro">
    <SectionTitle>
INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> Most contemporary grammar models employed in computational linguistics separate statements about dominance from those that determine linear precedence.</Paragraph>
    <Paragraph position="1"> The approaches for encoding linear precedence (LP) statements differ along several dimensions.</Paragraph>
    <Paragraph position="2"> Depending on the underlying grammatical theory, different criteria are employed in formulating ordering statements. Ordering constraints may be expressed by referring to the category, grammatical function, discourse r61e, and many other syntactic, semantic, morphological or phonological features.</Paragraph>
    <Paragraph position="3"> Depending on the grammar formalism, different languages are used for stating the constraints on permissible linearizations. LP rules, first proposed by Gazdar and Pullum (1982) for GPSG, are used, in different guises, by several contemporary grammar formalisms. In Functional Unification Grammar (Kay 1985) and implemented versions of Lexical Functional Grammar, pattern languages with the power of regular expressions have been utilized.</Paragraph>
    <Paragraph position="4"> Depending on the grammar model, LP statements apply within different ordering domains. In most frameworks, such as GPSG and HPSG, the ordering domains are local trees. Initial trees constitute the ordering domain in ID/LP TAGS (Joshi 1987). In current LFG (Kaplan &amp; Zaenen 1988), functional precedence rules apply to functional domains. Reape Research for this paper was mainly carried out in the project LILOG supported by IBM Germany. Some of the research was performed in the project DISCO which is funded by the German Federal Ministry for Research and Technology under Grant-No.: ITW 9002.</Paragraph>
    <Paragraph position="5"> We wish to thank our colleagues in SaarbriJcken, three anonymous referees and especially Mark Hepple for their valuable comments and suggestions.</Paragraph>
    <Paragraph position="6"> (1989) constructs word order domains by means of a special union operation on embedded tree domains.</Paragraph>
    <Paragraph position="7"> It remains an open question which choices along these dimensions will turn out to be most adequate for the description of word order in natural language.</Paragraph>
    <Paragraph position="8"> In this paper we do not attempt to resolve the linguistic issue of the most adequate universal treatment of word order. However we will present a method for integrating word order constraints in a typed feature unification formalism without adding new formal devices.</Paragraph>
    <Paragraph position="9"> Although some proposals for the interaction between feature unification and LP constraints have been published (e.g. Seiffert 1991), no encoding has yet been shown that integrates LP constraints in the linguistic type system of a typed feature unification formalism. Linguistic processing with a head-driven phrase structure grammar (HPSG) containing LP constraints has not yet been described in the literature. Since no implemented NL system has been demonstrated so far that handles partially free word order of German and many other languages in a satisfactory way, we have made an attempt to utilize the formal apparatus of HPSG for a new approach to processing with LP constraints. However, our method is not bound to the formalism of HPSG.</Paragraph>
    <Paragraph position="10"> In this paper we will demonstrate how LP constraints can be incorporated into the linguistic type system of HPSG through the use of parametrized types. Neither additional operations nor any special provisions for linear precedence in the processing algorithm are required. LP constraints are applied through regular unification whenever the head combines with a complement or adjunct.</Paragraph>
    <Paragraph position="11"> Although we use certain LP-relevant features in our examples, our aproach does not hinge on the selection of specific linguistic criteria for constraining linear order.</Paragraph>
    <Paragraph position="12"> Since there is no conclusive evidence to the contrary, we assume the simplest constraint language for formulating LP statements, i.e., binary LP constraints. For computational purposes such constraints are compiled into the type definitions for grammatical categories.</Paragraph>
    <Paragraph position="13"> With respect to the ordering domain, our LP constraints differ from the LP constraints commonly assumed in HPSG (Pollard &amp; Sag 1987) in that they  apply to nonsibling constituents in head domains.</Paragraph>
    <Paragraph position="14"> While LP constraints control the order of nodes that are not siblings, information is accumulated in trees in such a way that it is always possible to detect a violation of an LP constraint locally by checking sibling nodes.</Paragraph>
    <Paragraph position="15"> This modification is necessary for the proper treatment of German word order. It is also needed by all grammar models that are on the one hand confined to binary branching structures such as nearly all versions of categorial grammar but that would, on the other hand, benefit from a notion of LP constraints.</Paragraph>
    <Paragraph position="16"> Our approach has been tested with small sets of LP constraints. The grammar was written and run in STUF, the typed unification formalism used in the project LILOG.</Paragraph>
  </Section>
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