'HOW "/0 RESTRICT AMBIGUITY 0F DIBCOURSE Barbara Dunin-K@plic z Institute of Informatics University Of Warsaw P.O. Box 1210 00-901 Warszawa, POLAND ABBT~CT second interpretation seems better motivated. "/he next step is to define syntactic We single out a class of prototypes i.e., discourse domain, denoted by S_, i.e., a set L) a class of constructions forcing the obligatory of discourse expressions referring to coreference or obligatory noncoreference. An individuals (set of individuals). The mapping essential feature of prototypes is their assigning individuals to syntactic expressions undistinctiveness. In this sense they are the will be ca/led the reference function and most natural and efficient mearis of denoted by R. F'orma/ly, R : S D 2 ID. communication in discourse. The non-application of prototype should be Example well motivated. This leads to the rule of (DI) John and Peter admire winter. They restricted choice stating that whenever it is are often skiing together. possible the application of a prototype should be preferred. SDI" - {"John", "Peter", "winter", "they '''~ The rule of the restricted choice suggests the general outline of interpreting ambiguous ID 1 - ~John, Peter, winter~ sentences, strictly speaking, the method of ordering admissible interpretations: those which ("John") - {John} can be equivalently expressed by means of a prototype are less probable. In other words, R ("Peter") - {Peterl the rule of the restricted choice can be regarded as some kind of mechanism ordering R ("they") ,, {John, Peter]" the hypotheses for computer/on. R ("winter") - {winter} INTRODUCTION We say that discourse expressions x and y a.re coreferencial, what we denote by xCy, The crucial problem in discourse analysis if and only if they refer to the same set of is the appropriate transposition of all individuals. expressions occurring in it, into reality (see, for instance, the framework provided by Kemp Formally, in (Kamp, 1981)). Even preliminary analysis shows that one real object can be identified for each x,y ~ S u xCy iff R(x)= R(y) by various surface constructions. This forces the necessity of dividing surface expressions It is readily verified that C is an into classes denoting identical individuals. equivalence relation. Obviously each equivalence class of C contains coreferentia/ The above problem can formally be stated expressions. The set of equivalence classes as follows. To each discourse D we assign of C will be ca/led the reference sta~te of some reality which can be understood as discourse and denoted by RSD. a set ID of individuals (semantic discourse domain) together with a set of relations Example defined on ID. The semantic discourse domain can be interpreted twofold: (D2) John took a knife. 1.o as a set of real objects i.e., objects ( RSD 2 ) " "~ knife '~-. existing in actual world; John 5~ ~"a 2 ° as a set of mental objects i.e., objects (D3) John took a knife. He hurt himself. existing in language user's mind. (RSD3) ~"JOhn:', "he", "himself'.~ Although the first interpretation is more natural, it leads to some ontological problems, knife"}. concerning the distinction of fictitious and non-fictitious entities. Since there is no such We can observe here that adding new distinction from linguistic perspective the utterance to the discour.~;e changes its 93 reference state. In this sense RSD is a Let us consider few instances of dynamic notion Let us note also that the prototypes. Because the ideas presented here problem of anaphora solution can be regarded are implemented for the Polish language, the as defining the relation C for the whole notion of prototype will be illustrated with discourse. a number of Polish sentences. An elided subject specific for inflexional languages can Both the speaker, while constructin~ be observed here. It is clenoted by (~ a discourse, and the hearer, while eunalysing Because elided subject expresses some it, try to achieve the identity of RSD at each step of the discourse. We argue in this aspects of thematic continuity, its interpretation seems to be an important step paper that to accomplish this effect, the during discourse analysis. English speaker has at his disposal (at each moment) translations of presented examples preserve a more restricted set of linguistic their syntactic shape. Unfortunately they are constructions than it seems intuitively. Let sometimes incorrect as English sentences. us notice that expressions belonging to one equivalence class have various syntactic (1.) Piotr obudzit si~1 ~)3.. podszec£~ do okna, shapes at different steps of discourse. It ca/'* ~2 ot~vorzy~ je i ~)3 wyskoczy{. be shown that the syntactic form of expressions at particular moments is not Peter woke up, ~I came to the window, accidential, i.e., elements of indicated •2 opened it and ~)3 jumped out. equivalence class are not interchangeable. Expressions: Peter, #I, ~2, ~)3 are coreferentiaL Another P R O'I'O'I'YP E S interpretation is unadm,ssible, in (I) we deal with obligatory coreference of expressions Recent discourse theories provide several (denoted by a - -- b). levels of larlguage analysis: morphological, syntactic, semantic and sometimes pragmatic. Each of these levels determines a (2) ~I~2 podszeci% do okna, characteristic set of notions and mechanisms. It is assumed here that the analysis of 82", utterance on each levels of language should 4~4 ~skoczy~ yield complete information obtainable by tools available on this level ~)1. Woke up, q)2 came to the window, Classical anaphor resolvers act on semantic level on discourse analysis. ~Are take q)3 opened it and #4 jumped out. the position that for inflexion al languages the coreference relation can be partially described on the syntactic level An essential feature of in (2), similarly as in (a) (co-ordinate this partial characteristic is defining the clauses) and in (3), (4) (subordinate coreference relation quite apart from Peal clauses) the only acceptable interpretation individuals, i.e. without specyfing the reference is explicitely showed. function. "Po fix some ideas let us consider an (3) Z~im Ca,~s~ea~2 zgasi~ ~wiat~o. utterance containing the noun phrases NPl, ..., NP . If there is no information regarding n Before 911 left, 912 turned the light off. coreference, all we can do is to assert that the coreference relation is included between the "minimal" relation, i.e., relation identified (4) ~].~_Z_gasi~ ~wiat~o, zanim....w~2 wyszedL by the unit equivalence classes NP~ , ..., NP and the mcLximal one, i.e. adm,Ring in 911 Turned the L~ght off, before ~2 left. one nclass all number-gender agreeable phrases. The next examples concern the obligatory We very seldom deal with such a situation noncoreference of expressions (denoted by in practice. Almost a/ways we can assign to an utterance a syntactic level information a+-b) stating obligatory- coreference or obliqatory noncoreference of some expressions. (5) Ona lubi ja~ The surface constructions carrying this kind of information with respect to pronouns She likes her. and zero pronouns (in the case of elided subject) will be called prototypes. Ln other words prototypes can be regarded as (6) (~ zapyta~ Piotra,'~czy Jan p6jdzie syntactic means forcing obligatory coreference or obligatory noncoreference between pronouns do teatru. or zero pronouns and other surface expressions. ¢ Asked Peter, whether John would go to the theatre. 94 (7) ¢ Usiad~ do stc~u, a .Tan naleuI: mu wina. interpretations with respect to their plausibility. From the set of possible interpretations of Sat at the table, und John poured a sentence, those that can be equivalently expressed by means of prototype, should be him out some wine. regarded as less plausible. The justification of this choice is clear: if the speaker wanted The above examples pose the question of to point out such an he would how the class of prototypes should be singled interpretation, naturally achieve it by applying a prototype. out. This problem can be solved by specifying a collection of rules concerning In view of the obove we can formulate the obligatory coreference and obligatory the rule of restricted choice. It states that noncoreference. The exact format of these whenever it is possible the application of rules is beyond the scope of this paper. a prototype should be preferred. For inflexional languages they depend on the It is irrportant to notice that the rule of type of considered sentence, the sentence- restricted choice can be viewed from the -level functions of considered phrases perspective both of the speaker contructing and their syntactic shape. As a simple the discourse and the hearer modelling it. example of such a rule let us consider the The speaker should apply prototypes basic criterion of excluding coreference: whenever it is possible. The hearer should take this fact into consideration. If the object is expressed by means of a reflexive pronoum, then it is coreferential Let us try to interpret the concrete with the subject; in other cases the sentences with the help of the rule of referential identily of the subject and object restricted choice. is excluded. (8) Zanim ~)I wyszed~, .Tan zgasi~ ~wiat{o. This criterion can be applied both for deterrninig coreferents of objects - blocking Before ~I leftmasc, .Tohn tumedmasc the subject, and in tesf/n~ the possible antecedents of the subject - blocking the the light off. objects. This is exactly the case we have in (5). There are two interpretations here: THE RULE OF' RESTRICTED CHOICE (9) Zanim zgasi wia o A conclusive criterion of being a prototype results from analysing a given sentence it% Before ~I left, John turned the light off. isolation. If it is possible to assert or to exclude the referential identity of some l expressions of the sentence, indepedently of (1.0) Zanim ~I w3zszed2, Jan zgasi{ ~wiat~o.