Ellipsis Resolution with Underspecified Scope

Ellipsis Resolution with Underspecified Scope Michael Schiehlen � Institute of Natural Language Processing Azenbergstr. 12 70174 Stuttgart Fed. Rep. of Germany mike@ims.uni-stuttgart.de Abstract The paper presents an approach to ellipsis resolution in a framework of scope under- specification (Underspecified Discourse Representation Theory). It is argued that the approach improves on previous pro- posals to integrate ellipsis resolution and scope underspecification (Crouch, 1995; Egg et al., 2001) in that application pro- cesses like anaphora resolution do not re- quire full disambiguation but can work directly on the underspecified representa- tion. Furthermore it is shown that the ap- proach presented can cope with the exam- ples discussed by Dalrymple et al. (1991) as well as a problem noted recently by Erk and Koller (2001). 1 Introduction Explicit computation of all scope configurations is apt to slow down an NLP system considerably. Therefore, underspecification of scope ambiguities is an important prerequisite for efficient processing. Many tasks, like ellipsis resolution or anaphora res- olution, are arguably best performed on a represen- tation with fixed scope order. An underspecification formalism should support execution of these tasks. This paper aims to upgrade an existing underspec- ification formalism for scope ambiguities, Under- specified Discourse Representation Theory (UDRT) (Reyle, 1993), so that both ellipsis and anaphora res- olution can work on the underspecified structures. � Many thanks for discussion and motivation are due to the colleagues in Saarbrücken. Several proposals have been made in the lit- erature on how to integrate scope underspecifica- tion and ellipsis resolution in a single formalism, e.g. Quasi-Logical Forms (QLF) (Crouch, 1995) and the Constraint Language for Lambda Structures (CLLS) (Egg et al., 2001). That work has primar- ily aimed at devising methods to untangle quanti- fier scoping and ellipsis resolution which often in- teract closely (see Section 6). To this end, descrip- tion languages have been modelled in which the dis- ambiguation steps of a derivation need not be exe- cuted but rather can be explicitly recorded as con- straints on the final structure. Constraints are only evaluated when the underspecified representation is finally interpreted. In contrast, UDRT aims at pro- viding a representation formalism that supports in- terpretation processes such as theorem proving and anaphora resolution. Understood in this sense, un- derspecification often obviates the need for com- plete disambiguation. Another consequence is, how- ever, that the strategy of postponing disambigua- tion steps is in some cases insufficient. A case in point is the phenomenon dubbed Missing An- tecedents by Grinder and Postal (1971), illustrated in sentence (1): One of the pronoun’s antecedents is overt, the other is supplied by ellipsis resolution. (1) Harry sank a destroyer � and so did Bill and they �� � both went down with all hands. (Grinder and Postal, 1971, 279) Most approaches to ellipsis and anaphora resolution, e.g. (Asher, 1993; Crouch, 1995; Egg et al., 2001), can readily derive the reading. But consider: (2) Harry sometimes reads a book about a sea- battle and so does Bill. They borrow those books from the library. …

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