Expectations in Incremental Discourse Processing

The way in which discourse features express connections back to the previous discourse has been described in the literature in terms of adjoining at the right frontier of discourse structure. But this does not allow for discourse features that express ezpectations about what is to come in the subsequent discourse. After characterizing these expectations and their distribution in text, we show how an approach that makes use of substitution as well as adjoining on a suitably defined right frontier, can be used to both process expectations and constrain discouse processing in general. 1 I n t r o d u c t i o n 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, "even" and "only"), and identification of those structures underlying a discourse that are associated with coherence relations between its units. In the course of developing an incremental approach to the latter, we noticed a variety of constructions in discourse that raise expectations about its f u t u r e 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. The following examples illustrate the creation of expectations through discourse markers: Example 1 a. On the one hand, John is very generous. b. On the other, he is extremely difficult to find. Example 2 a. On the one hand, John is very generous. b. On the other, suppose you needed some money. c. You'd see that he's very difficult to find. Example 3 a. On the one hand, John is very generous. b. For example, suppose you needed some money. c. You would just have to ask him for it. b. On the other hand, he is very difficult to find. Example 1 illustrates the expectation that, following a clause marked "on the one hand", the discourse will express a constrasting situation (here marked by "on the other"). 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. Tha t 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 "suppose". Subordinate conjunctions (e.g., "just as", "although", "when", etc.) can lead to similar expectations when they appear in a preposed subordinate clause eg. 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. 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-