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<Paper uid="W90-0104">
  <Title>A New Model for Lexical Choice for Open-Class Words</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1. Introduction
</SectionTitle>
    <Paragraph position="0"> Lexical choice for open-class words has typically been regarded as a matching or classification problem. The generation system is given a semantic structure that represents an object or event, and a dictionary that represents the semantic meanings of the lexical units (Zgusta 1971) of the target language; it then chooses the lexical unit (or set of lexical units) that best matches the object or event. This paper proposes an alternative lexical choice architecture, in which the lexical choice process is regarded as a constraint satisfaction problem: the generation system must choose a lexical unit that is accurate (truthful), valid (conveys the necessary informarion), and preferred (maximal under a preference function). 1 This constraint-based architecture is more robust than classification systems. In parricular, it allows a clean separation to be made between what the system knows of the object or event, and what the system wishes to communicate about the object or event; and it allows lexical choices to be biased towards basic-level (Rosch 1978) and other preferred lexical units.</Paragraph>
    <Paragraph position="1"> Throughout this paper, it will be assumed that both lexical units and objects/events are represented as t Currently at the Department of Artificial Intelligence, University of Edinburgh, 80 South Bridge, Edinburgh EH1 1HN, Scotland. E-maih reitel@aitma.edinburgh.ac.uk I This paper does not exmnine the kind of oollocational and selectional constraints discussed by Cumming (1986) and Nirenburg and Nirenburg (1988).</Paragraph>
    <Paragraph position="2"> classes in a KL-ONE type taxonomy (Brachman and Schmolze 1985). For example, the lexical unit Bachelor might be represented as the generic class (Human with role value restrictions Sex:Male, Agestatus:Adult, Married:False); and the object Terry might be represented as the individual class (Human with role fillers Sex:Male, Eye-color:Brown, Birthplace:Chicago, Employer:IBM .... ). Default attributes as well as definitional information can be associated with lexical units; this is essential for making appropriate lexical choices (Section 5). Figure 1 shows a sample taxonomy that will be used for most of the examples in this paper.</Paragraph>
    <Paragraph position="3"> Lexical units (e.g., Bachelor) are shown in bold font, while objects (e.g., Terry) are shown in italic font.</Paragraph>
    <Paragraph position="4"> Role value restrictions (VR's), such as Sex:Male for Man, are fisted textually instead of displayed graphically, to simpfify the complexity of the diagram; default attributes (e.g., Can-fly:True for Bird) are listed in italic font. Basic-level classes (e.g., Man) are underlined.</Paragraph>
    <Paragraph position="5"> Section 2 of the paper discusses classification-based systems and some of the problems associated with them.</Paragraph>
    <Paragraph position="6"> Section 3 introduces the proposed constraint-based system; Section 4 looks in more detail at the lexical preferences used by the system; and Section 5 briefly discusses the need for default atlributes in the semantic representations of lexical units. The constraint-based lexical choice system has been incorporated into the FN system (Reiter 1990), which generates certain kinds of natural language object descriptions. FN uses some additional preference rules that primarily affect NP formarion; these rules are not discussed in this paper.</Paragraph>
  </Section>
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