A Three-Level Model for Plan Exploration

In modeling the structure of task-related discourse using plans, it is important to distinguish between plans that the agent has adopted and is pursuing and those that are only being considered and explored, since the kinds of utterances arising from a particular domain plan and the patterns of reference to domain plans and movement within the plan tree are quite different in the two cases. This paper presents a three-level discourse model that uses separate domain and exploration layers, in addition to a layer of discourse metaplans, allowing these distinct behavior patterns and the plan adoption and reconsideration moves they imply to be recognized and modeled. DISCOURSE MODEL LEVELS In task-related discourse, much of the discourse structure derives directly from the task structure, so that a model of the agent's plans can serve as a useful discourse model, with discourse segment boundaries mapping to sub-plan boundaries and the like. This simple model works well in applications like expert-apprentice dialogues, where a novice agent is currently pursuing a single domain plan. Since the discourse tracks the domain plan so closely in such cases, it is fairly easy to make the links between the agent's queries and the relevant domain plans. But this single-level model is not rich enough to handle phenomena like clarification subdialogues or plan revision, as seen in the work of Litman, Carberry, and others. Litman's model [Lit85, LA87, LA90] employs a stack of discourse mctaplans on top of the base domain plan, producing a two-level model that can handle clarification subdialogues and other discourse phenomena that go beyond the domain plans. Carberry [Carg0] adds an independent stack of discourse goals, for similar reasons. In earlier work [Ram89a], I explored the use of a different kind of metaplan to model what I called the problem-solving level, where the agent is exploring possible plans, rather than pursuing an adopted one. Such plan exploration contexts, which can include comparison between alternative plans or consideration of plans in hypothetical circumstances, are quite different from adopted domain plan execution contexts, both in terms of the reference patterns to domain plans and in terms of the kinds of utterances that are generated. This paper describes an effort to combine these earlier approaches into a three-level model where the discourse metaplans can be rooted in either the exploration or domain plan levels, so that both kinds of plan-related behavior can be modeled. Such a model can capture the differences between the plan exploration level and the domain level in terms of the appropriate plan recognition and query generation processes, thus broadening the range of discourse phenomena that can be modeled. It also allows us to model shifts between levels, as the agent explores, adopts, and reconsiders particular plans. T H E N A T U R E OF P L A N E X P L O R A T I O N Cohen and Levesque [CLg0] have recently pointed out the theoretical problems that arise, while using a planning system to model a rational agent, from failing to distinguish between a system's plans and its intentions, since agents frequently form plans that they never adopt. It is this same distinction that motivates the division proposed here of the domain-plan-related portion of the discourse model into separate levels representing first those domain plans and goals which the agent has adopted and is pursuing, here called the domain