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<Paper uid="P97-1012">
  <Title>Expectations in Incremental Discourse Processing</Title>
  <Section position="3" start_page="0" end_page="88" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Discourse processing subsumes several distinguishable but interlinked processes. These include reference and ellipsis resolution, inference (e.g., inferential processes associated with focus particles such as, in English, &amp;quot;even&amp;quot; and &amp;quot;only&amp;quot;), and identification of those structures underlying a discourse that are associated with coherence relations between its units.</Paragraph>
    <Paragraph position="1"> In the course of developing an incremental approach to the latter, we noticed a variety of constructions in discourse that raise expectations about its future structural features. We found that we could represent such expectations by adopting a lexical variant of TAG - LTAG (Schabes, 1990) - and using its substitution operation as a complement to adjoining. Perhaps more interesting was that these expectations appeared to constrain the subsequent discourse until they were resolved. This we found we could model in terms of constraints on adjoining and substitution with respect to a suitably defined Right Frontier. This short paper focuesses on the phenomenon of these expectations in discourse and their expression in a discourse-level LTAG. We conelude the paper with some thoughts on incremental discourse processing in light of these expectations.</Paragraph>
    <Paragraph position="2"> The following examples illustrate the creation of expectations through discourse markers: Example 1  a. On the one hand, John is very generous.</Paragraph>
    <Paragraph position="3"> b. On the other, he is extremely difficult to find.</Paragraph>
    <Paragraph position="4"> Example 2 a. On the one hand, John is very generous.</Paragraph>
    <Paragraph position="5"> b. On the other, suppose you needed some money.</Paragraph>
    <Paragraph position="6"> c. You'd see that he's very difficult to find.</Paragraph>
    <Paragraph position="7"> Example 3 a. On the one hand, John is very generous.</Paragraph>
    <Paragraph position="8"> b. For example, suppose you needed some money.</Paragraph>
    <Paragraph position="9"> c. You would just have to ask him for it.</Paragraph>
    <Paragraph position="10"> b. On the other hand, he is very difficult to find.</Paragraph>
    <Paragraph position="11">  Example 1 illustrates the expectation that, following a clause marked &amp;quot;on the one hand&amp;quot;, the discourse will express a constrasting situation (here marked by &amp;quot;on the other&amp;quot;). Examples 2 and 3 illustrate that such an expectation need not be satisfied immediately by the next clause: In Example 2, clause (b) partially resolves the expectation set up in (a), but introduces an expectation that the subsequent discourse will indicate what happens in such cases.</Paragraph>
    <Paragraph position="12"> That expectation is then resolved in clause (c). In Example 3, the next two clauses do nothing to satisfy the expectation raised in clause (a): rather, they give evidence for the claim made in (a). The expectation raised in (a) is not resolved until clause (d). These examples show expectations raised by sentential adverbs and the imperative use of the verb &amp;quot;suppose&amp;quot;. Subordinate conjunctions (e.g., &amp;quot;just as&amp;quot;, &amp;quot;although&amp;quot;, &amp;quot;when&amp;quot;, etc.) can lead to similar expectations when they appear in a preposed subordinate clause - eg.</Paragraph>
    <Paragraph position="13"> Example 4 a. Although John is very generous, b. if you should need some money, c. you'd see that he's difficult to find.</Paragraph>
    <Paragraph position="14"> As in Example 2, clause 4(a) raises the expectation of learning what is nevertheless the case. Clause 4(b) partially satisfies that expectation by raising a hy- null pothetical situation, along with the expectation of learning what is true in such a situation. This latter expectation is then satisfied in clause 4(c).</Paragraph>
    <Paragraph position="15"> In summary, these expectations can be characterized as follows: (1) once raised, an expectation must be resolved, but its resolvant can be a clause that raises its own expectations; (2) a clause raising an expectation can itelf be elaborated before that expectation is resolved, including elaboration by clauses that raise their own expectations; and (3) the most deeply &amp;quot;embedded&amp;quot; expectations must always be resolved first.</Paragraph>
    <Paragraph position="16"> Now these are very likely not the only kinds of expectations to be found in discourse: Whenever events or behavior follow fairly regular patterns over time, observers develop expectations about what will come next or at least eventually. For example, a dialogue model may embody the expectation that a suggestion made by one dialogue participant would eventually be followed by an explicit or implicit rejection, acceptance or tabling by the other. Other dialogue actions such as clarifications or justifications may intervene, but there is a sense of an expectation being resolved when the suggestion is responded to.</Paragraph>
    <Paragraph position="17"> Here we are focussed on discourse at the level of individual monologue or turn within a larger discourse: what we show is that discourse manifests certain forward-looking patterns that have similar constraints to those of sentence-level syntax and can be handled by similar means. One possible reason that these particualr kinds of expressions may not have been noticed before is that in non-incremental approaches to discourse processing (Mann and Thompson, 1988; Marcu, 1996), they don't stand out as obviously different.</Paragraph>
    <Paragraph position="18"> The labels for discourse coherence relations used here are similar to those of RST (Mann and Thompson, 1988), but for simplicity, are treated as binary. Since any multi-branching tree can be converted to a binary tree, no representational power is lost. In doing this, we follow several recent converging computational approaches to discourse analysis, which are also couched in binary terms (Gardent, 1997; Marcu, 1996; Polanyi and van den Berg, 1996; Schilder, 1997; van den Berg, 1996).</Paragraph>
    <Paragraph position="19"> Implicit in our discussion is the view that in processing a discourse incrementally, its semantics and pragmatics are computed compositionally from the structure reflected in the coherence relations between its units. In the figures presented here, non-terminal nodes in a discourse structure are labelled with coherence relations merely to indicate the functions that project appropriate content, beliefs and other side effects into the recipient's discourse model. This view is, we believe, consistent with the more detailed formal interfaces to discourse semantics/pragmatics presented in (Gardent, 1997; Schilder, 1997; van den Berg, 1996), and also allows for multiple discourse relations (intentional and informational) to hold between discourse units (Moore and Pollack, 1992; Moser and Moore, 1995; Moser and Moore, 1996) and contribute to the semantic/pragmatics effects on the recipient's discourse model.</Paragraph>
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