Accommodating Context Change

ACCOMMODATING CONTEXT CHANGE Bonnie Lynn Webber and Breck Baldwin Department of Computer and Information Science University of Pennsylvania Philadelphia, PA 19104-6389 Interact: {bonnie~central,breck@linc}.cis.upenn.edu* ABSTRACT Two independent mechanisms of context change have been discussed separately in the literature - context change by entity introduction and context change by event simulation. Here we discuss their integration. The effectiveness of the integration de- pends in part on a representation of events that cap- tures people's uncertainty about their outcome - in particular, people's incomplete expectations about the changes effected by events. We propose such a representation and a process of accommodation that makes use of it, and discuss our initial implementa- tion of these ideas. Introduction Consider the following example: Example 1 John made a handbag from an inner-tube. a. He sold it for twenty dollars. b. *He sold them for fifty dollars. c. He had taken it from his brother's car. d. Neither of them was particularly useful. Here two entities are introduced via indefinite noun phrases (NPs) in the first sentence. The alternative follow-ons (a-d) show that subsequent reference to those entities is constrained. In particular, (b) high- lights the difference in their existential status, even though there is no syntactic difference in how they are introduced. Now consider *This work was partially supported by ARO grant DAAL 03-89-C-0031, DARPA grant N00014-90-J-1863, and NSF grant IRI 90-16592 to the University of Pennsylvania. The paper draws upon material first presented at the workshop on Defensible Reasoning in Semantics and Pragmatics held at the European Summer School on Logic, Language and Infor- mation, Saarbr~cken, Germany, August 1991. Example 2 Mix the flour, butter and water. a. Knead the dough until smooth and shiny. b. Spread the paste over the blueberries. c. Stir the batter until all lumps are gone. In each of the alternative follow-on (a-c), a different definite NP refers to the result of the mixing, even though the terms "dough", "paste" and "batter" are not interchangeable. (They denote substances with different consistencies, from a pliant solid - dough - to a liquid - batter.) In both these examples, events 1 are mentioned that change the world being described. These exam- ples will be used to show why the two mechanisms of context change discussed separately in the litera- ture (context change by entity introduction and con- text change by event simulation) must be integrated (Section 2). For such integration to be effective, we argue that it must be based on a representation of events that captures people's uncertainty about their outcome - in particular, people's incomplete expec- tations about the changes effected by events. An un- derstanding system can then use these expectations to accommodate [15] the particular changes that are mentioned in subsequent discourse (Section 3). In Section 4, we discuss our initial implementation of these ideas. This work is being carried out as part of a project (AnlmNL) aimed at creating animated task simu- lations from Natural Language instructions [2; 4; 5; 6; 7; 14; 20]. Instructions are a …

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