A validation-structure-based theory of plan modification and reuse

The ability to modify existing plans to accommodate a variety of externally imposed constraints (such as changes in the problem specification, the expected world state, or the structure of the plan) is a valuable tool for improving efficiency of planning by avoiding repetition of planning effort. In this paper, we present a theory of incremental plan modification suitable for hierarchical nonlinear planning, and describe its implementation in a system called PRIAR. In this theory, the causal and teleological structure of the plans generated by a planner are represented in the form of an explanation of correctness called the "validation structure". Individual planning decisions are justified in terms of their relation to the validation structure. Plan modification is formalized as a process of removing inconsistencies in the validation structure of a plan when it is being reused in a new or changed planning situation. The repair of these inconsistencies involves removing unnecessary parts of the plan and adding new nonprimitive tasks to the plan to establish missing or failing validations. The result is a partially reduced plan with a consistent validation structure, which is then sent to the planner for complete reduction. We discuss this theory, present an empirical evaluation of the resulting plan modification system, and characterize the coverage, efficiency and limitations of the approach.