Case-based reasoning [23, 1, 32] is a problem solving methodology based on using a library of solutions for similar problems, i.e., a library of "cases" with their respective solutions. Roughly speaking, case-based planning consists into storing generated plans and using them for finding new plans [15, 8, 29]. In practice, what is stored is not only a specific problem with a specific solution, but also some additional information that is considered useful to the aim of solving new problems, e.g., information about how the plan has been derived [30], why it works [20, 16], when it would not work [17], etc. Different case-based planners differ on how they store cases, which cases they retrieve when the solution of a new problem is needed, how they adapt a solution to a new problem, whether they use one or more cases for building a

The paradigms of transformational planning, case-based planning, and plan debugging all involve a process known as plan adaptation - modifying or repairing an old plan so it solves a new problem. In this paper we provide a domain-independent algorithm for plan adaptation, demonstrate that it is sound, complete, and systematic, and compare it to other adaptation algorithms in the literature. Our approach is based on a view of planning as searching a graph of partial plans. Generative planning starts at the graph's root and moves from node to node using plan-refinement operators. In planning by adaptation, a library plan - an arbitrary node in the plan graph - is the starting point for the search, and the plan-adaptation algorithm can apply both the same refinement operators available to a generative planner and can also retract constraints and steps from the plan. Our algorithm's completeness ensures that the adaptation algorithm will eventually search the entire graph and its systematicity ensures that it will do so without redundantly searching any parts of the graph.