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<Paper uid="E89-1025">
  <Title>A rich environment for experimentation with unification grammars</Title>
  <Section position="2" start_page="0" end_page="0" type="metho">
    <SectionTitle>
IDSIA, Lugano
ABSTRACT
</SectionTitle>
    <Paragraph position="0"> This paper describes some of the features of a sophisticated language and environment designed for experimentation with unification-oriented linguistic descriptions. The system, which is called ud, has to date been used successfully as a development and prototyping tool in a research project on the application of situation schemata to the representation of real text, and in extensive experimentation in machine translation. While the ud language bears close resemblances to all the well-known unification grammar formalisms, it offers a wider range of features than any single alternative, plus powerful facilities for notational abstraction which allow users to simulate different theoretical approaches in a natural way.</Paragraph>
    <Paragraph position="1"> After a brief discussion of the motivation for implementing yet another unification device, the main body of the paper is devoted to a description of the most important novel features of ud.</Paragraph>
    <Paragraph position="2"> The paper concludes with a discussion of some questions of implementation and completeness. null several languages: principally a demanding machine translation exercise and a substantial investigation into some practical applications of situation semantics (Johnson, Rosner and Rupp, forthcoming).</Paragraph>
    <Paragraph position="3"> The interaction between users and implementers has figured largely in the development of the system, and a major reason for the richness of its language and environment has been the pressure to accommodate the needs of a group of linguists working on three or four languages simultaneously and importing ideas from a variety of different theoretical backgrounds.</Paragraph>
    <Paragraph position="4"> Historically ud evolved out of a near relative of PATR-II (Shieber, 1984), and its origins are still apparent, not least in the notation. In the course of development, however, ud has been enriched with ideas from many other sources, most notably from LFG (Bresnan, 1982) and HPSG (Sag and Pollard, 1987).</Paragraph>
    <Paragraph position="5"> Among the language features mentioned in the paper are a wide range of data types, including lists, trees and user-restricted types, in addition to the normal feature structures i. Introduction.</Paragraph>
    <Paragraph position="6"> The development of ud arose out of the need to have available a full set of prototyping and development tools for a number of different research projects in computational linguistics, all involving extensive text coverage in comprehensive treatment of disjunction dynamic binding of name segments  path-A particular article of faith which has been very influential in our work has been the conviction that well-designed programming languages (including ones used primarily by - 182linguists), should not only supply a set of primitives which are appropriate for the application domain but should also contain within themselves sufficient apparatus to enable the user to create new abstractions which can be tuned to a particular view of the data.</Paragraph>
    <Paragraph position="7"> We have therefore paid particular attention to a construct which in ud we call a relational abstraction, a generalisation of PATR-II templates which can take arguments and which allow multiple, recursive definition. In many respects relational abstractions resemble Prolog procedures, but with a declarative semantics implemented in terms of a typical feature-structure unifier.</Paragraph>
    <Paragraph position="8"> are intended to be read as subscripts.</Paragraph>
    <Paragraph position="9"> Three other special symbols are used: + stands for the unification operator * stands for top, the underdefined element.</Paragraph>
    <Paragraph position="10"> # stands for bottom, the overdefined element that corresponds to failure.</Paragraph>
    <Paragraph position="11"> The semantics of unification proper are summarised in figures 2 - 4. Clauses \[i\] - \[3\] define its algebraic properties; clauses \[4\] - \[6\] define unification over constants, lists and trees in a manner analagous to that found in Prolog.</Paragraph>
    <Paragraph position="12"> i.i. Structure of the paper Section 2 gives a concise summary of the semantics of the basic ud unifier. This serves as a basis for an informal discussion, in Section 3, of our implementation of relational abstractions in terms of 'lazy' unification. The final section contains a few remarks on the issue of completeness, and a brief survey of some other language features.</Paragraph>
  </Section>
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