In 1976, Newell and Simon [Newell and Simon1976] proposed that intelligent behavior arises from the manipulation of symbols--entities that represent other entities, and that the process by which intelligence arises is heuristic search. Search is a process of formulating and examining alternatives. It starts with an initial state, a set of candidate actions, and criteria for identifying the goal state. It is often guided by heuristics, or rules of thumb,'' which are generally useful, but not guaranteed to make the best choices. Starting from the initial state, the search process selects actions to transform that state into new states, which themselves are transformed into more new states, until a goal state is generated.