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<Paper uid="C96-1084">
  <Title>Bridging Textual Ellipses</Title>
  <Section position="2" start_page="0" end_page="496" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Text phenomena, e.g., textual forms of ellipsis and anaphora are a challenging issue for the design of parsers for text understanding systems, since lacking recognition lacilities either result in referentially incoherent or invalid text knowledge representations.</Paragraph>
    <Paragraph position="1"> At the conceptual level, textual ellipsis (also called functional anaphora) relates a quasi-anaphoric expression to its extrasentential antecedent by conceptual attributes (or roles) associated with that antecedent (see, e.g., the relation between &amp;quot;Ladezeit&amp;quot; (charge time) and &amp;quot;Al&amp;u&amp;quot; (accumulator) in (lc) and (lb)).</Paragraph>
    <Paragraph position="2"> Thus, it complements the phenomenon of nominal anaphora, where an anaphoric expression is related to its antecedent in terms of conceptual generalization (as, e.g., &amp;quot;'Rechner&amp;quot; (computer) refers to &amp;quot;'316LT&amp;quot;, a particular notebook, in (lb) and (la)). The resolution of text-level nominal (and pronominal) anaphora contributes to the construction of referentially valid text knowledge bases, while the resolution of textual ellipsis yields referentially coherent text knowledge bases. Both phenomena tend to interact, as evidenced by the examples below. &amp;quot;Akku&amp;quot; (accumulator) in (lb) is a nominal anaphor referring to &amp;quot;Nickel-Metallllydride-Akku&amp;quot; (nickel-metal-hydride accumulator) in (1 a), which, when resolved, provides the proper referent for relating &amp;quot;Ladezeit&amp;quot; (charge time) in (lc) to it. In the case of textual ellipsis, the missing conceptual link between two discourse elements occurring in adjacent utterances must be inferred in order to establish the local coherence of the discourse (for an early statemerit of that idea, cf. Clark (1975)). In sentence (lc) the information is missing that &amp;quot;Ladezeit&amp;quot; (charge time) links up with &amp;quot;Akku&amp;quot; (accumulator). This relation can only be made explicit if conceptual knowledge about the domain, viz. the relation charge-time-of between the concepts CIIARGE-TIMF, and ACCUMU-LATOR, is available.</Paragraph>
    <Paragraph position="3"> The solution we propose is embedded within the centering model (Grosz et al., 1995). In this approach, discourse entities linking successive utterances ~in a discourse segment are organized in terms of centers.</Paragraph>
    <Paragraph position="4"> Local coherence in discourse is established whenever a center element of the previous utterance is associated with an expression that has a valid semantic inteq~retation (i.e., is realized) in the following utterance. Textual ellipsis has only been given insufficient treatment within the centering model in terms of rather sketchy realization conditions as opposed to the more elaborated constraints for (pro)nominal anaphora. The heuristics we propose include language-independent conceptual criteria and language-dependent information structure constraints reflecting the context-boundedness or unboundedness of discourse elements within the considered utterances.</Paragraph>
  </Section>
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