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Dependence on Outcomes and Superfluous Contingencies

The fact that a contingency plan assumes a particular outcome of an uncertainty means only that it cannot depend upon a different outcome of that uncertainty. Cassandra does not enforce any constraint that the plan must causally depend upon the outcome that it assumes. For instance, in the example described in Section 2.3.2, the plan to take Ashland does not actually depend on Western being blocked; it could be executed successfully regardless of the level of traffic on Western.

This observation raises an interesting question: If a plan for a contingency turns out not to depend on any outcome of the uncertainty that gave rise to it, would this not obviate the need for plans for alternative contingencies? For instance, in our example, it might seem sensible to execute the plan to use Ashland regardless of whether Western is blocked. It might thus seem that the planner should edit the plan in some way so as to eliminate apparently superfluous contingencies. However, it can easily be shown that a version of the plan that does not involve dependence on any outcome of the uncertainty will be generated elsewhere in the search space. In the example, this would mean that the planner would in fact consider a plan that simply involved taking Ashland. If the search heuristics penalize plans involving contingencies appropriately this other plan should be preferred to the contingency plan, all other things being equal.

Louise Pryor <>;
Last modified: Mon Mar 18 18:04:19 1996