Planning under Uncertainty in Dynamic Domains - Carnegie Mellon ...

3.1. Overview of prodigy 4.0 23Barge(barge1)PlaceDock(Richmond)Sea-Sector(west-coast)Figure 3.3: The type hierarchy for the planning domain. Objects in the exampleplanning problem are shown in brackets below their type.1. Move-Bargebarge1Richmond west-coastPump-Oilbarge1west-coastMove-Bargebarge1west-coastRichmondUnload-Oilbarge1RichmondRootHead PlanTail PlanFigure 3.4: A candidate plan for the planning problem to dispose of the oil. Thevariable names have been ommitted from the instantiated operators. The head plancontains one step, to move the barge from Richmond to the west coast.at the top of the partial order. If the head plan is applied to the initial state, it leadsto a new state called the current state. This current state is used to guide planningin two ways. First, no steps are added to the tail plan to achieve subgoals that arealready achieved in the current state. Second, steps are preferred if they have fewerpreconditions that are not true in the current state. Both of these guidance rules areheuristics, and may delay nding a solution. They typically speed the search process,however. Figure 3.4 shows an example candidate plan that might be generated in thesearch for a solution to the current problem.prodigy 4.0 begins its search with the null plan, in which the head plan isempty and the tail plan consists of the single node Root. It can expand a candidateplan in one of two ways: it can either add a new step to the tail plan to achievesome open precondition, or it can move a step from the tail plan to the head plan.A step that is moved to the head plan must be applicable in the current state. Itis added to the end of the head plan, yielding a new current state. More details onthe representation of plans and the search techniques used in prodigy 4.0 can befound in [Fink & Veloso 1995; Veloso et al. 1995; Veloso & Stone 1995].For example, in Figure 3.4, the step with operator Pump-Oil is applicable andcould be moved to the head plan. Moving it would yield a new current state,