Now that we're back in the swing of things, let's continue with nondeterminism (specifically AND/OR graph-search); see problem 4.6:
Explain precisely how to modify the AND/OR graph-search algorithm to generate a cyclic plan if no acyclic plan exists. You will need to deal with three issues labeling the plan steps so that a cyclic plan can point back to an earlier part of the plan, modifying OR-search so that it continues to look for acyclic plans after finding a cyclic plan, and augmenting the plan representation to indicate whether a plan is cyclic. Show how your algorithm works on (a) the slippery vacuum world, and (b) the slippery, erratic vacuum world. You might wish to use a computer implementation to check your results.
This will come in handy when we deal with adversarial search in chapter 5.