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<?xml version="1.0" standalone="yes"?> <Paper uid="P89-1030"> <Title>DISCOURSE ENTITIES IN JANUS</Title> <Section position="4" start_page="244" end_page="244" type="metho"> <SectionTitle> 3 Context Dependence of Discourse Entities </SectionTitle> <Paragraph position="0"> A formal semantics was assumed though not given for the sample logical language used by Webbar. The initial descriptions (IDs) of DEs produced by her rules were stated in this language too, and thus are meant to denote the object the DE represents.</Paragraph> <Paragraph position="1"> For example, the rule which applies to the representation for independent definite NPs assigns to the resulting DE an ID which is the representation itself: (t x S (P x)) => ID: (t x S (P x)) where t is Russell's iota operator. Thus, the ID for &quot;the cat&quot; in &quot;1 saw the cat&quot; is (t x cats T). (Since the body of the t in this example has no additional predication on x, it is merely T, for TRUE.) However, because IDs are solely drawn from the meaning representation of the isolated text, they may not suffice to denote a unique object. Connection to prior discourse knowledge or information from further discourse may be necessary to establish a unique referent, or determining the referent may not even be necessary. For example, the ID for &quot;the cat&quot; would need to be evaluated in a context where there is only one salient cat in orddr to obtain a denotation.</Paragraph> <Paragraph position="2"> Our system's representation of a DE is a structure containing several fields. The &quot;logical-form&quot; field contains a WML expression which denotes the object the DE'describes (this corresponds roughly to Webber's ID). Given that WML is intensional, we are able to explicitly represent context dependence by having the logical form include an intensional core, plus tense, time, and world information (which includes discourse context) that grounds the intension so that it may be evaluated. For example, the logical form for the DE corresponding to &quot;the cat&quot; in our system is</Paragraph> <Paragraph position="4"> where time, if unfilled, defaults to the present, and world defaults to the real world and current discourse state. The semantics of our IOTA operator makes it denotationless if there is not exactly one salient object that fits the description in the context, else its denotation is that unique object. In our interactive system each reference needs to be fully resolved to be used successfully. If unknown information is necessary to obtain a unique denotation for a IOTA term, a simple clarification dialogue should ensue. (Clarification is not implemented yet, currently the set of all values fitting the IOTA is used.) An example using the time index is the noun phrase &quot;the ships that were combat ready on 12/1/88&quot;, which would generate a DE with logical form:</Paragraph> <Paragraph position="6"> Representing this time index in the logical form is crucial, since a later reference to it, made in a different time context must still denote the original object. For example, &quot;Are they deployed?&quot; must have &quot;they&quot; refer to the ships that were combat ready on 12/1/88, not at the time of the latter utterance.</Paragraph> <Paragraph position="7"> In order to derive the proper time and world context for the discourse entities, we added structural rules that recognize intensional and index-binding logical contexts. Our DE generation algorithm uses these rules to gather the necessary information as it recurses into the logical representation (applying rules as it goes) so that when a regular rule fires on a language construct, the appropriate outer-scoping time/world bindings will get used for the generated DEs.</Paragraph> <Paragraph position="8"> It should be noted that, as the discussion above suggests, a definite NP always gives rise to a new discourse entity in our system. If it is determined to be anaphoric, then a pointer to the DE it co-refers with (when found) will be added to its &quot;refers-to&quot; field, indicating they both denote the same object.</Paragraph> </Section> <Section position="5" start_page="244" end_page="245" type="metho"> <SectionTitle> 4 DEs for Independent Indefinite NPs </SectionTitle> <Paragraph position="0"> In Webber's work, the initial description (ID) for a DE stemming from an independent existential (i.e., with no dependencies on an outer FORALL quantifier), contained an EVOKE predicate. &quot;1 saw a cat&quot;: (EXISTS x cat8 (maw I x)) would generate a DE with ID:</Paragraph> <Paragraph position="2"> &quot;The cat I saw that was evoked by sentence Sent&quot;, where Sent is the parsed clause for '1 saw a cat&quot;.</Paragraph> <Paragraph position="3"> The purpose of EVOKE was to make clear that although more than one cat may have been seen, the &quot;a&quot; picks out one in particular (which one we do not know except that it is the one mentioned in the utterance), and this is the cat which makes the EVOKE true. Any subsequent reference then picks out the same cat because it will access this DE. The semantics of the EVOKE predicate and the type of the S argument (which is syntactic in nature) were unclear, so we looked for a different formulation with better understood semantics.</Paragraph> <Paragraph position="4"> Predicate logic already provides us with a mechanism for selecting arbitrary individuals from the domain via skolem functions (used as a mechanism for removing existentials from a formula while preserving satisfiability). Skolem functions have been used in computational linguistics to indicate quantifier scope, for example (VanLehn, 1978). Following a suggestion by R. Scha, we use skolem functions in the logical form of the DE for the &quot;indefinite individuals&quot; introduced by independent existentials (Scha et al., 1987). For clarity and consistency with the rest of the language, we use a sortedskolem form, where the range of the function is specified. Since we use this for representing existentials that are independent, the function has no arguments and is thus equivalent to a sorted constant whose denotation is undetermined when introduced. (In this sense it is consistent with Karttunen's (1976) and Kamp's (1984) view of the indefinite's role as a referential constant, but unlike Kamp, here the sentence's meaning representation is separate from the representation of the evoked entity.) Thus we introduced a new operator to WML named SKOLEM, for expressions of the form (SKOLEM n <sort>), where n is an integer that gets incremented for each new skolem created, as a way of naming the skolem function. For the example above, the core logical form (stripping the outer intension and indices) for the DE of &quot;a cat&quot; would be: (SKOL~M I (SET x oats (saw I x))) denoting a particular cat from the set of aJl the cats I saw. The type of a SKOLEM expression is well-defined and is given by the following type rule:</Paragraph> <Paragraph position="6"> where INTEGERS is the type for integers, and (SETS a) is the type of sets whose members have type a.</Paragraph> <Paragraph position="7"> This type rule says that when the first argument of SKOLEM is of type INTEGER, and the second is a set with elements of type a, then the type of the SKOLEM expression is a. Therefore, the type of the above example is cats. The explicit connection to the originating sentence which the EVOKE predicate provided is found in our scheme outside of the logical representation by having a pointer in the DE's structure to the parse tree NP constituent, and to the structure representing the communicative act performed by the utterance (in the fields &quot;corresponding-constituent&quot; and &quot;originating-communicative-act&quot;, respectively). These connections are used by the pronoun resolut/on algorithms which make use of syntactic information. null Does the denotation of a skolem constant ever get determined? In narrative, and even in conversation, identifying the individual referred to by the indefinite NP frequently doesn't occur. However, in our interactive system, each reference must be fully resolved. When the evaluation component of Janus determines a successful value to use for the existential in the text's logical form, the appropriate function denotation for SKOLEM n gets defined, and the &quot;extension&quot; field is set for the discourse entity.</Paragraph> <Paragraph position="8"> Note that many interesting issues come up in the treatment of reference to these indefinite entities in a real system. For example, cooperative responses by the system introduce new entities that must be taken into account. If the user asks &quot;Is there a carrier within 50 miles of Hawaii?&quot;, a cooperative &quot;There are two: Constellation and Kennedy&quot; (as opposed to just &quot;Yes&quot;) must add those two carriers as entities, which now overshadow the singular skolem entity for &quot;a carder within 50 miles of Hawaii&quot;. On the other hand, a &quot;No&quot; answer should block any further reference to the carrier skolem, since its denotation is null, while still allowing a reference to a class entity derived from it, as in &quot;Is there one near San Diego?&quot; where one refers to the class carriers.</Paragraph> <Paragraph position="9"> The treatment presented works for straightforward cases of independent indefinites. Trickier cases like donkey sentences (Kamp, 1984, Webber, 1981) and interactions with negation have not yet been addressed. null</Paragraph> </Section> <Section position="6" start_page="245" end_page="247" type="metho"> <SectionTitle> 5 Dependent NPs </SectionTitle> <Paragraph position="0"/> <Section position="1" start_page="245" end_page="247" type="sub_section"> <SectionTitle> 5.1 Dependent Indefinite NPs </SectionTitle> <Paragraph position="0"> Our work uncovered a need for modifications in Webber's structural rules for quantifiers from indefinite and definite NPs which have dependencies on variables bound directly or indirectly by an outer FORALL quantifier. In this section we address the case of dependent existentials arising from indefinite NPs.</Paragraph> <Paragraph position="1"> We first argue that the predicate EVOKE is not needed in this context. Then we point out the need for generalizing the rule to take into account not just FORALL, but all scoping operators that intervene between the outer FORALL and the inner EXISTS.</Paragraph> <Paragraph position="2"> Finally, we show that the dependencies between discourse entities must be explicitly maintained in the logical forms of newly created DEs that depend on them.</Paragraph> <Paragraph position="3"> Webber's rules are designed to apply from the outermost quantifier in; each time a rule is applied the remaining logical form is modified to be in terms of the just created DE. For example, &quot;Every boy saw a girl he knows&quot; has logical form (for the bound pronoun reading):</Paragraph> <Paragraph position="5"> The first step is to apply the rule for an independent universal quantifier: R0: (FORALL x S (P x)) => de: S This application yields the entity for &quot;the set of all</Paragraph> <Paragraph position="7"> and we rewrite the logical form to be:</Paragraph> <Paragraph position="9"> The steps shown so far are consistent with both Webber's and our approach. Now we want to apply the general rule for existentials within the body of a distributive, in order to generate an entity for the relevant set of girls. Webber uses Rule 3 in (Webber, 1983) (here corrected to position the existential's sort S inside the scope of the outer quantifiers in the where FORALL Yl&quot;&quot;Yk is shorthand for FORALL Yl de 1 (...(FORALL Yk dek, analogously for EXISTS, and S or P depends directly or indirectly on Yl &quot;&quot;Yk' Now the first DE we want to generate with this rule is for &quot;the set of girls, each of which is known by some boy in DE 1, and was seen by him&quot;. Does each girl in the set also have to satisfy an EVOKE predicate? It seems that any future reference back to the set formed by the existential seeks to obtain a/I items fitting the description, not some subset constrained by EVOKE. For example, if the example above is followed by &quot;the girls tried to hide&quot;, taking &quot;the girls&quot; anaphorically, one wants a/I the girls seen by some boy in DE 1 that knows them, no less. Our core logical representation for the set of girls is thus: where EVOKE has been removed, and the DE's sort field is S t for the &quot;root type&quot; of S, which is the type of the members of S, in order to appropriately constrain the DE's sort (instead of leaving it as the unconstrained &quot;things&quot;).</Paragraph> <Paragraph position="10"> A second change that needs to be made is to generalize the left hand side of the rule so that the scoping expressions outscoping the inner EXISTS in the pattern also be allowed to include other scoping operators, such as EXISTS and IOTA. As long as the outermost quantifier is a FORALL, any other dependent scoping expression within it will generate a set-denoting DE and will behave as a distributive environment as far as any more deeply embedded expressions are concerned. In other words, the distributiveness chains along the dependent quantifiers. To see this, consider the more embedded example &quot;Every boy gave a girl he knew a peach she wanted&quot;, where there is an intervening existential between the outer FORALL and innermost EXISTS. The core logical form for this sentence is:</Paragraph> <Paragraph position="12"> DE 1 would be as above. Using rule R3' DF_. 2 becomes: null</Paragraph> <Paragraph position="14"> &quot;The set of girls, each of which is known by some boy in DE 1, and got a peach she wanted from that boy.&quot; Now the peach quantifier should generate a set DE in terms of DE 1 and DE 2. Applying R3' gives us:</Paragraph> <Paragraph position="16"> &quot;The set of peaches z such that there is a girl in DE 2 (who is known by some boy in DE I, and who got some peach she wan.tpd from the boy), who wants z, and who got it from some boy in DE 1''.</Paragraph> <Paragraph position="17"> Now a third and final problem becomes apparent: for the general case of arbitrary embedding of dependent quantifiers we generate a DE (e.g., DF_,3) dependent on other DEs from the outer quantifiers, but the dependencies between those DEs (e.g., DE 1 and DE2) are not maintained. This is counter-intuitive, and also leads to an under-specified set DE. In the peaches example above, envision the situation where a boy b I gave out two peaches Pl and P2 : one to a girl gl he knew, and one to a girl g2 he didn't know, who also got a peach P3 from another boy b 2 who did know her. These are the facts of interest in this scenario: I. (& (gava b I gl p1) (know b I gl)</Paragraph> <Paragraph position="19"> Since b 1 and b 2 are in DE 1 (due to facts 1 and 3), and g2 is in DE 2 (due to fact 3), then P2 is in DE 3 (due to fact 2 and according to the DF_. 3 logical form above).</Paragraph> <Paragraph position="20"> But P2 should notbe in DE 3, since P2 was NOT given to a girl by a boy she knew. The set of peaches obtained for DE 3 is too large. The problem would not arise if in the DE 3 logical form, the variables ranging over DF-- 2 were appropriately connected to DE 1 using the dependent restriction present in the original formula (knows xy). A correct DE 3 is:</Paragraph> <Paragraph position="22"> (gave x y z))))) To be able to do this, the rule-application algorithm must be modified to include the restriction information (for dependent restrictions) when the formula gets rewritten in terms of a newly created DE. Therefore the final generalized rule, which includes other scoping operators and works on properly connected DEs is as follows: where S or P depend directly or indirectly on v 1...v n, Qi may be FORALL, EXISTS, or IOTA, and the scoping operators outside the inner EXISTS have already been processed by any appropriate rules that have replaced their original sorts by the Sis, which are in terms of generated DEs and explicitly show any DE dependencies. The right hand side is as before, with existentials picking out elements from each outer quantifier.</Paragraph> <Paragraph position="23"> act. Since &quot;them&quot; and *it&quot; have different number requirements, there is no ambiguity and the anaphor resolution module resolves &quot;them&quot; to the DE corresponding to &quot;the C1 carriers in the Indian Ocean&quot; and &quot;it&quot; to the DE for Kennedy. We are currently working on having system-initiated actions also generate entities.</Paragraph> </Section> </Section> <Section position="7" start_page="247" end_page="247" type="metho"> <SectionTitle> 7 Conclusions and Further Work </SectionTitle> <Paragraph position="0"> Webber's general approach to discourse entity generation from a logical representation proved very useful in our efforts. We were able to recast her basic ideas in our logical framework, and currently use the generated DEs extensively.</Paragraph> <Paragraph position="1"> The fact that the generation of DEs is done via structural rules operating on a semantic representation provided a degree of modularity that allowed our pronoun resolution component to work automatically when we combined a new syntactic component with our semantic and discourse component (replacing an ATN by a unification grammar, in an independently motivated experiment). We are currently starting to port the DE generation component to the BBN Spoken Language System (Boisen et al., 1989), and plan to integrate it with the intra-sentential mechanisms in (Ingria and Stallard, 1989). The fact that entity representations are mostly semantic in nature, not syntactic, also facilitated the addition and use of non-linguistic entities in a uniform way.</Paragraph> <Paragraph position="2"> There are several areas that we would like to study to extend our current treatment. We want to address the interactions between centering phenomena and non-linguistic events that affect discourse focus, such as changing contexts via a menu selection in an expert system.</Paragraph> <Paragraph position="3"> Our paraphrasing component (Meteer and Shaked, 1988) already uses the discourse entities to a limited extent. One area of future work is to have the language generator make more extensive use of them, so it can smoothly refer to focused objects.</Paragraph> <Paragraph position="4"> Finally, although quantified expressions are already generated in Janus for events implicit in many verbs, they are not being used for DEs. We would like to address the problem of event reference and its interaction with temporal information, using ideas such as those in (Webber, 1988) and in the special issue of ComputationaJ Linguistics on tense and aspect (Vol. 14, Number 2 June 1988).</Paragraph> </Section> <Section position="8" start_page="247" end_page="247" type="metho"> <SectionTitle> 8 Acknowledgments </SectionTitle> <Paragraph position="0"> The work presented here was supported under DARPA contract #N00014-85-C-0016. The views and conclusions contained in this document are those of the author and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the Defense Advanced Research Projects Agency or of the United States Government. The author would like to thank Dave Stallard for invaluable discussions during the writing of this paper. Thanks also to Remko Scha, Lance Ramshaw, Ralph Weischedel, and Candy Sidner.</Paragraph> </Section> class="xml-element"></Paper>