Threat-Removal Strategies for Partial-Order Planning

McAllester and Rosenblitts' (1991) systematic nonlinear planner (SNLP) removes threats as they are discovered. In other planners such as SIPE (Wilkins, 1988), and NOAH (Sacerdoti, 1977), threat resolution is partially or completely delayed. In this paper, we demonstrate that planner efficiency may be vastly improved by the use of alternatives to these threat removal strategies. We discuss five threat removal strategies and prove that two of these strategies dominate the other three--resulting in a provably smaller search space. Furthermore, the systematicity of the planning algorithm is preserved for each of the threat removal strategies. Finally, we confirm our results experimentally using a large number of planning examples including examples from the literature.