Encapsulation of Actions and Plans in Conditional Planning Systems

We introduce a notion of macroaction for a given subplan. A macroaction is an action representing the given plan and it can be considered as an abstraction, neglecting the internal structure of the original plan. The description of the macroaction is sufficiently powerful to represent all the information needed for synchronization and clobbering detection of the given subplan. It is possible to prove that when substituting in a correct plan a macroaction with its real composition, the overall plan remains correct. It is pointed out that it is possible to build large libraries of plans by means of techniques of plan compilation and manage them with fast-selection and plan re-use methods. A comparison of different kind of macroactions is done in the frameworks of linear, non-linear and conditional planning. It is shown that a conditional macroaction is required even in the case of nonlinear unconditional plans. A conditional planning model based on conditional formula calculus is used. Conditional formulae represent uncertain situations and denote sets of mutually exclusive states. The method of compilation is defined through a weakest precondition semantics for conditional plans.