Searches through state space and returns a conditional plan for the given problem. The conditional plan is a list of either an action or an if-then construct (consisting of a list of states and consequent actions). The final product, when printed, resembles the contingency plan on page 134. This function is equivalent to the following on page 136:
function AND-OR-GRAPH-SEARCH(problem) returns a conditional plan, or failure OR-SEARCH(problem.INITIAL-STATE, problem, [])
@param problem
@return a conditional plan or null on failure