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<Paper uid="C86-1156">
  <Title>Discourse, anaphora and parsing</Title>
  <Section position="3" start_page="669" end_page="670" type="intro">
    <SectionTitle>
(4) ClherlCifff*C
</SectionTitle>
    <Paragraph position="0"> where f is the reference marker associated with her A sequence of discourse contexts is well-formed for a string if all of the relations associated with the lexieal items in the string hold; i.e. the discourse contexts arc a solution to the relational equations. Sometimes these equations wiil have a single solution; in that case, the discourse is unambiguous.</Paragraph>
    <Paragraph position="1"> However, usually the equations have multiple solutions, which means, in effect, that the discourse has many interpretations.</Paragraph>
    <Paragraph position="2"> This arises, in the present discussion, when a pronoun has several possible antecedents. 3 On the other hand, it is also possible that the equations have no solution at all. This case arises when a pronoun is used in a discourse context that contains no appropriate reference marker at all.</Paragraph>
    <Paragraph position="3"> At a more abstract level, we can view this model as one in which the context is a stream of reference markers, which is threaded from one lexical item to the next. The equations associated with individual iexical items act as (possibly nondeterministic) operators on their input stream to produce an output stream, which serves as the input to the following lexieel item.</Paragraph>
    <Paragraph position="4"> One of the main virtues of this simple picture is that it invites comparison with other ideas. Our proposed notion of meaning is clearly reminiscent of the claim in (Barwise and Perry 1983) that meaning is a relation between different types of situation, though it also has its roots in earlier work on indexical semantics, such as (Stalnaker 1972). Second, it is also 2 It seems that this technique of factoring a single non-monotonic representation into a series of monotonic ones is applicable in many areas other than the one discussed here.</Paragraph>
    <Paragraph position="5"> At an abstract level it is similar to the technique discussed by (Kowalski 1979a). It is also similar to the use of difference lists in logic programming, since the &amp;quot;content&amp;quot; of a particular element is the difference bewteen its &amp;quot;output&amp;quot; and its &amp;quot;input&amp;quot;. s In such a case, our program merely enumerates all possible interpretations, which results in the familiar combinatorial explosion of solutions. A better technique, which we cannot explore here, would be factor out the ambiguity and localize it in the representation.</Paragraph>
    <Paragraph position="6">  reminiscent of the technique used in logic programming known as difference lists (Pereira 1985) or threading.</Paragraph>
  </Section>
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