VT: An Expert Elevator Designer That Uses Knowledge-Based Backtracking

Even least commitment systems such as MOLGEN (Stefik 1981a, 1981b) are sometimes forced to guess. In the course of designing genetics experiments, MOL-GEN tries to avoid making a decision until all constraints that might affect the decision are known. In some cases, this postponement is not possible, and the system becomes stuck; none of the pending decisions can be made with complete confidence. In such a case, a decision based on partial information is needed, and such a decision might be wrong. In this case, a problem solver needs the ability either to backtrack to correct bad decisions or to maintain parallel solutions corresponding to the alternatives at the stuck decision point However, if alternative guesses exist at each point, and there are many such decision points on each solution path, a commitment to examine every possible combination of alternatives proves unwieldy.