A Metaplan Model for Problem-Solving Discourse

A METAPLAN MODEL FOR PROBLEM-SOLVING DISCOURSE* Lance A. Ramshaw BBN Systems and Technologies Corporation 10 Moulton Street Cambridge, MA 02138 USA ABSTRACT The structure of problem-solving discourse in the expert advising setting can be modeled by adding a layer of metaplans to a plan-based model of the task domain. Classes of metaplans are introduced to model both the agent's gradual refinement and instantiation of a domain plan for a task and the space of possible queries about preconditions or fillers for open variable slots that can be motivated by the exploration of par- ticular classes of domain plans. This metaplan structure can be used to track an agent's problem-solving progress and to predict at each point likely follow-on queries based on related domain plans. The model is implemented in the Pragma system where it is used to suggest cor- rections for ill-formed input. 1. INTRODUCTION Significant progress has been achieved recently in natural language (NL) understanding systems through the use of plan recognition and "plan tracking" schemes that maintain models of the agent's domain plans and goals. Such sys- tems have been used for recognizing discourse structure, processing anaphora, providing cooperative responses, and interpreting intersen- tential ellipsis. However, a model of the dis- course context must capture more than just the plan structure of the problem domain. Each dis- course setting, whether argument, narrative, cooperative planning, or the like, involves a level of organization more abstract than that of domain plans, a level with its own structures and typical strategies. Enriching the domain plan model with a model of the agent's plans and strategies on this more abstract level can add "This research was supported by the Advanced Research Projects Agency of the Department of Defense and was monitored by ONR under Contract No. N00014-85- C-0016. The views and conclusions contained in this docu- ment are those of the author and should not be interpreted as necessarily representing the official policies, either ex- pressed or implied, of the Defense Advanced Research Projects Agency or the U.S. Government. significant power to an NL system. This paper presents an approach to pragmatic modeling in which metaplans are used to model that level of discourse structure for problem-solving dis- course of the sort arising in NL interfaces to expert systems or databases. The discourse setting modeled by metaplans in this work is expert-assisted problem-solving. Note that the agent's current task in this context is creating a plan for achiev- ing the domain goal, rather than executing that plan. In problem-solving discourse, the agent poses queries to the expert to gather information in order to select a plan from among the various possible plans. Meanwhile, in order to respond to the queries cooperatively, the expert must maintain a model of the plan being considered by the agent. Thus the expert is in the position of deducing from the queries that are the agent's observable behavior which possible plans the agent is currently considering. The metaplans presented here model both the agent's plan- building choices …

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