When an error occurs in the execution of a plan, it is necessary to request a replanning. At this point the method is very simple and straight forward. It involves indicating which operation has failed, and restarting the planning algorithm of the previous section. The assignment of operation numbers to the plan the planner uses requires that a lookup table be used for converting operation numbers to operation names, so that the planner would know which operation failed. It is also of use to point out that in this case backward planning causes some problems when it tries to replan. This is based on the fact that when it is selecting rules to ‘undo’ features the planner has started from the complete part, backtracking would suggest backtracking toward the completed part (in the opposite direction from the direction needed for replanning). For example, we might say a drilling operation has failed half way through production. The planner would note that the operation has failed, and select another replacement operation, and another feature is undone, and so on down another path in the operation tree until all features are undone. In effect we are finding another path to the start point. But, what we need for practical purposes is the exact opposite, a path to the start state (the final product).