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<Paper uid="P89-1030">
  <Title>DISCOURSE ENTITIES IN JANUS</Title>
  <Section position="3" start_page="0" end_page="244" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> 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 &amp;quot;Every senior in Milford High School has a car&amp;quot; gives rise to at least 3 entities, describable in English as &amp;quot;the seniors in Milford High School&amp;quot;, &amp;quot;Milford High School&amp;quot;, and &amp;quot;the set of cars each of which is owned by some senior in Milford High School&amp;quot;. These entities may then be accessed by the following next utterances, respectively: &amp;quot;They graduate in June.&amp;quot; &amp;quot;It's a good school.&amp;quot; &amp;quot;They completely fill the parking lot.&amp;quot; Webber (1978, 1983) addressed the question of determining what discourse entities are introduced by a text. She defined rules which produce &amp;quot;initial descriptions&amp;quot; (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.</Paragraph>
    <Paragraph position="1"> The present work recasts her rules in Janus's intensional logic framework (described in section 2).</Paragraph>
    <Paragraph position="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.</Paragraph>
    <Paragraph position="3"> In the following sections, we first present the representational requirements for this approach, and introduce our logical language (section 2).</Paragraph>
    <Paragraph position="4"> 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.</Paragraph>
    <Paragraph position="5"> 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.</Paragraph>
    <Paragraph position="6"> 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.</Paragraph>
    <Paragraph position="7"> Finally, section 7 concludes and presents plans for future work.</Paragraph>
    <Paragraph position="8"> This is, to our knowledge, the first implementation of Webber's DE generation ideas. We designed the  algorithms and structures necessary to generate discourse entities from our logical representation of the meaning of utterances, and from pointing gestures, and currently use them in Janus's (Weischedel et al., 1987, BSN, 1988) pronoun resolution component, which applies centering techniques (Grosz et al., 1983, Sidner 1981, 1983, Brennan et al. 1987) to track and constrain references. Janus has been demonstrated in the Navy domain for DARPA's Fleet Command Center Battle Management Program (FCCBMP), and in the Army domain for the Air Land  Webber found that appropriate discourse entities could be generated from the meaning representation of a sentence by applying rules to the representation that are strictly structural in nature, as long as the representation reflects certain crucial aspects of the sentence. This has the attractive feature that any syntactic formalism may be used if an appropriate semantic representation is produced. Some of the requirements (described in (Webber 1978, 1983)) on the representation are: (1) it must distinguish between definite and indefinite NPs and between singular and plural NPs, (2)it must specify quantifier scope, (3) it must distinguish between distributive and collective readings, (4)it must have resolved elided verb phrases, and (5) it must reflect the modifier structure of the NPs (e.g., via restricted quantification). An important implied constraint is that the representation must show one recognizable construct (a quantifier, for example) per DE-invoking noun phrase. These constructs are what trigger the DE generation rules.</Paragraph>
    <Paragraph position="9"> Insofar as a semantic representation reflects all of the above in its structure, structural rules will suffice for generating appropriate DEs, but otherwise information from syntax or other sources may be necessary. There is a trade-off between using a level of representation that shows the required distinctions, and the need to stay relatively close to the English structure in order to only generate DEs that are justiffed by the text. For example, in Janus, in addition to quantiflers from NPs, the semantic representation has quantiflers for verbs (events), and possibly extra quantifiers introduced in representing deeper meaning or by the collective/distributive processing. Therefore, we check the syntactic source of the quantifiers to ensure that we only generate entities for quantifiers that arose from NPs (using the bound variable as an index into the parse tree).</Paragraph>
    <Paragraph position="10"> Other than the caveat just discussed, the Janus meaning representation language WML (for World Model Language) (Hinrichs et al., 1987) meets all the other constraints for DE generation. WML is a higher-order intensional language that is based on a synthesis between the kind of language used in PHLIQA (Scha, 1976) and Montague's Intensional Logic  (Montague, 1973). A newer version of WML (Stallard, 1988) is used in the 8BN Spoken Language System (Boisen et al., 1989). The intensionality of WML makes it more powerful than the sample language Webber used in developing her structural rules.</Paragraph>
    <Paragraph position="11"> The scoping expressions in WML have a sort field (which restricts the range of the variable) and have the form: (1= x s (P x)) where B is a quantifier such as FORALL or EXISTS, a term-forming operator like IOTA or SET, or the lambda abstraction operator LAMBDA. S is the sort, a set-denoting expression of arbitrary complexity specifying the range of x, and (P x) is a predication in terms of x. The formal semantics of WML assigns a type to each well-formed expression which is a function of the types of its parts. If expression E has type T, the denotation of E, given a model M and a time t and world w, is a member of the set which is T's domain. One use of types in our system is for enforcing selectional restrictions. The formation rules of WML, its type system, and its recursive denotation definition provide a formal syntax and semantics for WML.</Paragraph>
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