Localizing Expression Of Ambiguity

In this paper we describe an implemented program for localizing the expression of many types of syntactic ambiguity, in the logical forms of sentences, in a manner convenient for subsequent inferential processing. Among the types of ambiguities handled are prepositional phrases, very compound nominals, adverbials, relative clatmes, and preposed prepositional phrases. The algorithm we use is presented, and several possible shortcomings and extensions of our method are discussed. 1 I n t r o d u c t i o n Ambiguity is a problem in any natural language processing system. Large grammars tend to produce large numbers of alternative analyses for even relatively simple sentences. Furthermore, as is well known, syntactic information may be insu~cient for selecting a best reading. It may take semantic knowledge of arbi t rary complexity to decide which alternative to choose. In the TACITUS project [Hobbs, 1986; Hobbs and Martin, 1987] we are developing a pragmatics component which, given the logical form of a sentence, uses world knowledge to solve various interpretation problems, the r~=oluti,JD of syntactic ambiguity among them. Sentences are translated into logical form by the DIALOGIC system for syntactic mid semantic analysis [Grosz et al., 1982]. In this paper we describe how information about alternative parses is passed concisely from DIALOGIC to the pragmatics component, and more generally, we discuss a method of localizing the representation of syntactic ambiguity in the logical form of a sentence. One possible approach to the ambiguity problem would be to produce a set of logical forms for a sentence, one for each parse tree, and to send them one at a time to the pragrnatics component. This involves considerable duplication of effort if the logical forms are largely the same and differ only with respect to attachment. A more efficient approach is to try to localize the information about the alternate possibilities. Instead of feeding two logical forms, which differ only with respect to an attachment site, to a pragraatics component, it is worthwhile trying to condense the information of the two logical forms together into one expression with a disjunction inside it representing the attachment ambiguity. That one expression may then be given to a pragmatics component with the effect that parts of the sentence that would have been processed twice are now processed only once. The savings can be considerably more dramatic when a set of five or ten or twenty logical forms can be reduced to one, as is often the case. In effect, this approach translates the syntactic ambiguity problem into a highly constrained coreference problem. It is as though we translated the sentence in (1) into the two sentences in (2) (1) John drove down the street in a car. (2) John drove down the street. It was in a car. where we knew "it" had to refer either to the street or to the driving. Since coreference is one of the phenomena the pragmatics component is designed to cope with [Hobbs and Martin, 1987], such a translation represents progress toward a solution. The rest of this paper describes the procedures we use to produce a reduced set of logical forms from a larger set. The basic strategy hinges on the idea of a neutral representation [Hobbs, 1982]. This is similar to the idea behind Church's Pseudo-attachment [Church, 19801 . Pereira's Rightmost Normal Form [Pereira, 1983], and what Rich et al. refer to as the Procr~tination Approach to parsing [Rich, Barnett, Wittenburg, and Whittemore. 1986]. However, by expressing the ambiguity as a disjunction in logical form, we put it into the form most convenient for subsequent inferential processing. 2 R a n g e o f P h e n o m e n a 2 . 1 Attachment Possibilities There are three representative classes of at tachment ambiguities, and we have implemented our approach :o each of these. For each class, we give representative examples and show the relevant logical form fragments that encode the set of possible attachments. In the first class are those constituents that may at tach to either nouns or verbs. (3) John saw the man with the telescope.