Plan Repair as an Extension of Planning

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In dynamic environments, agents have to deal with changing situations. In these cases, repairing a plan is often more efficient than planning from scratch, but existing planning techniques are more advanced than existing plan repair techniques. Therefore, we propose a straightforward method to extend planning techniques such that they are able to repair plans. This is possible, because plan repair consists of two different operations: (i) removing obstructing constraints (such as actions) from the plan, and (ii) adding actions to achieve the goals. Adding actions is similar to planning, but as we demonstrate, planning heuristics can also be used for removing constraints, which we call unrefinement. We present a plan repair template that reflects these two operations, and we present a heuristic for unrefinement that can make use of an arbitrary existing planning technique. We apply this method to an existing planning system (VHPOP) resulting in POPR, a plan repair system that performs much better than replanning from scratch, and also significantly better than another recent plan repair method (GPG). Furthermore, we show that the plan repair template is a generalisation of existing plan repair methods.