Localizing Expression of Ambiguity

Localizing Expression of Ambiguity John Bear and Jerry R. Hobbs Artificial Intelligence Center SRI International Abstract In this paper we describe an implemented program for localizing the expression of many types of syntactic am- biguity, 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 exten- sions of our method are discussed. 1 Introduction 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 se- mantic knowledge of arbitrary 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 alter- native parses is passed concisely from DIALOGIC to the pragmatics component, and more generally, we discuss a method of localizing the representation of syntactic ambi- guity 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 dupli- cation of effort if the logical forms are largely the same and differ only with respect to attachment. A more effi- cient 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 com- ponent, 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 ambigu- ity. That one expression may then be given to a pragmat- ics 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 re- duced to one, as is often the case. In effect, this approach translates the syntactic ambigu- ity 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 …

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