When rules are examined by the inference engine, actions are executed if the information supplied by the user satisfies the conditions in the rules. Two methods of inference often are used, forward and backward chaining. Forward chaining is a top-down method which takes facts as they become available and attempts to draw conclusions (from satisfied conditions in rules) which lead to actions beingexecuted. Backward chining is the reverse. It is a bottom-up procedure which starts with goals (or actions) and queries the user about information which may satisfy the conditions contained in the rules. If is a verification process rather then an exploration process. An example of backward chaining is MYCIN [vMS81], and an example of forward chaining is Expert [WK81]. A system which uses both is Prospector [DGH79]