Explicit causal structure representations have been widely used in classical planning systems to guide a variety of aspects of planning, including plan generation, modification and generalization. For the most part, these representations were limited to single-contributor causal structures. Although widely used, single-contributor causal structures have several limitations in handling partially ordered and partially instantiated plans. The foremost among these is that they are incapable of exploiting redundancy in the plan. In this paper, we explore multi-contributor causal structures as a way of overcoming these limitations. We will provide a general formulation for multi-contributor causal links, and explore the properties of several special classes of this formulation. We will also discuss their applications in plan generation, modification and generalization.

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ion and respecialization is a general structural adaptation technique that abstracts the piece of the retrieved solution, and respecializes it later. Respecialization results in applying other specializations of the abstractions to the current situation. Thus, it results in analogical problem solving. This technique is used in PLEXUS (Alterman, 1986) as a second step in adaptation process (first, a null adaptation is used, and only if something fails, the system replans it using abstraction and respecialization). A similar approach is used in PERSUADER (Sycara, 1987). 4. Critic-based adaptation is a structural adaptation based on using critics to debug almost correct solutions (Simmons, 1988; Hammond, 1989a; Gonzalez and Laureano-Ortiz, 1992). A critic checks if a particular combination of features can cause a problem in a plan. If such a feature is found, a specific repair strategy is applied for repair. In CHEF, several criticbased adaptation rules are used, e.g., deletion of unneces...