Discourse Entities in Janus

This paper addresses issues that arose in applying the model for discourse entity (DE) generation in B. Webber's work (1978, 1983) to an interactive multimodal interface. Her treatment was extended in 4 areas: (1)the notion of context dependence of DEs was formalized in an intensional logic, (2)the treatment of DEs for indefinite NPs was modified to use skolem functions, (3)the treatment of dependent quantifiers was generalized, and (4) DEs originating from non-linguistic sources, such as pointing actions, were taken into account. The discourse entities are used in intraand extra-sentential pronoun resolution in BBN Janus. 1 I n t r o d u c t i o n Discourse entities (DEs) are descriptions of objects, groups of objects, events, etc. from the real world or from hypothesized or possible worlds that are evoked in a discourse. Any communicative act, be it spoken, written, gestured, or system-initiated, can give rise to DEs. As a discourse progresses, an adequate discourse model must represent the relevant entities, and the relationships between them (Grosz and Sidner, 1986), A speaker may then felicitously refer anaphorically to an object (subject to focusing or centering constraints (Grosz et al., 1983, Sidner 1981, 1983, Brennan et al. 1987) ) if there is an existing DE representing it, or if a corresponding DE may be directly inferred from an existing DE. For example, the utterance "Every senior in Milford High School has a car" gives rise to at least 3 entities, describable in English as "the seniors in Milford High School", "Milford High School", and "the set of cars each of which is owned by some senior in Milford High School". These entities may then be accessed by the following next utterances, respectively: "They graduate in June." "It's a good school." "They completely fill the parking lot." Webber (1978, 1983) addressed the question of determining what discourse entities are introduced by a text. She defined rules which produce "initial descriptions" (IDs) of new entities stemming from noun phrases, given a meaning representation of a text. An ID is a logical expression that denotes the corresponding object and uses only information from the text's meaning representation. The declarative nature of Webber's rules and the fact that they relied solely on the structure of the meaning representation, made her approach well suited for implementation. The present work recasts her rules in Janus's intensional logic framework (described in section 2). Two goals guided our approach: (1)that our DE representations be semantically clear and correct according to the formal definitions of our language, and (2) that these representations be amenable to the processing required in an interactive environment such as ours, where each reference needs to be fully resolved against the current context. In the following sections, we first present the representational requirements for this approach, and introduce our logical language (section 2). Then we discuss issues that arose in trying to formalize the logical representation of DEs with respect to (1) the context dependence of their denotations, and (2) the indeterminacy of denotation that arises with indefinite NPs. For context dependence, we use an intensional logic expression indexed by time and world indices (discussed in section 3). This required us to extend Webber's rules to detect modal and other index-binding contexts. In representing DEs for indefinites (appearing as existential formulae in our meaning representation), we replaced Webber's EVOKE predicate with skolem constants for the independent case, where it does not contain a variable bound by a higher FORALL quantifier (section 4), and do not use EVOKE at all in the dependent case. In section 5 we introduce a generalized version of the rules for generating DEs for dependent quantifiers stemming from indefinite and definite NPs which overcomes some difficulties in capturing dependencies between discourse entities. In our multi-modal interface environment, it is important to represent the information on the computer screen as part of the discourse context, and allow references to screen entities that are not explicitly introduced via the text input. Section 6 briefly discusses some of these issues and shows how pointing actions are handled in Janus by generating appropriate discourse entities that are then used like other DEs. Finally, section 7 concludes and presents plans for future work. This is, to our knowledge, the first implementation of Webber's DE generation ideas. We designed the