As long as the world behaves exactly as the action descriptions describe it, then executing a plan in the ideal or incomplete-information cases will always result in goal achievement. As each step is executed, the world state will be as predicted—as long as nothing goes wrong. "Something going wrong" means that the world state after an action is not as predicted. More specifically, the remaining plan segment will fail if any of its preconditions is not met. The preconditions of a plan segment (as opposed to an individual step) are all those preconditions of the steps in the segment that are not established by other steps in the segment. It is straightforward to annotate a plan at each step with the preconditions required for successful completion of the remaining steps. 

The required conditions are just the propositions protected by all the causal links beginning at or before the current step and ending at or after it. Then we can detect a potential failure by comparing the current preconditions with the state description generated from the percept sequence. This is the standard model of execution monitoring, first used by the original STRIPS planner. STRIPS also introduced the triangle table, an efficient representation for fully annotated plans. 

A second approach is to check the preconditions of each action as it is executed, rather than checking the preconditions of the entire remaining plan. This is called action monitoring. As well as being simpler and avoiding the need for annotations, this method fits in well with realistic systems where an individual action failure can be recognized. For example, if a robot agent issues a command to the motor subsystem to move two meters forward, the subsystem can report a failure if the robot bumps into an obstacle that materialized unexpectedly. On the other hand, action monitoring is less effective than execution monitoring, because it does not look ahead to see that an unexpected current state will cause an action failure some time in the future. For example, the obstacle that the robot bumped into might have been knocked off the table by accident much earlier in the plan. 

An agent using execution monitoring could have realized the problem and picked it up again. Action monitoring is also useful when a goal is serendipitously achieved. That is, if someone or something else has already changed the world so that the goal is achieved, action monitoring notices this and avoids wasting time by going through the rest of the plan. These forms of monitoring require that the percepts provide enough information to tell if a plan or action is about to fail. In an inaccessible world where the relevant conditions are not perceivable, more complicated strategies are needed to cope with undetected but potentially serious deviations from expectations. This issue is beyond the scope of the current chapter. We can divide the causes of plan failure into two kinds, depending on whether it is possible to anticipate the possible contingencies:

0 Bounded indeterminacy: In this case, actions can have unexpected effects, but the possible effects can be enumerated and described as part of the action description axiom. For example, the result of opening a can of paint can be described as the disjunction of having paint available, having an empty can, or spilling the paint. Using a combination of CPOP and the "D" (disjunctive) part of POP-DUNC we can generate conditional plans to deal with this kind of indeterminacy.

Unbounded indeterminacy: In this case, the set of possible unexpected outcomes is too large to be completely enumerated. This would be the case in very complex and/or dynamic domains such as driving, economic planning, and military strategy. In such cases, we can plan for at most a limited number of contingencies, and must be able to replan when reality does not behave as expected.

 The next subsection describes a simple method for replanning based on trying to get the plan "back on track" as quickly as possible. Section 13.3 describes a more comprehensive approach that deals with unexpected conditions as an integral part of the decision-making process.

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