Postponing Threats in Partial-Order Planning

An important aspect of partial-order planning is the resolution of threats between actions and causal links in a plan. We present a technique for automatically deciding which threats should be resolved during planning, and which should be delayed until planning is otherwise complete. In particular we show that many potential threats can be provably delayed until the end; that is, if the planner can find a plan for the goal while ignoring these threats, there is a guarantee that the partial ordering in the resulting plan can be extended to eliminate the threats. Our technique involves: 1) construction of an operator graph that captures the interaction between operators relevant to a given goal, 2) decomposition of this graph into groups of related threats, and 3) postponement of threats with certain properties.