An apparatus and method for debugging a forward-chaining (rule- based) expert system are disclosed. A single STEP command controls the expert system and automatically presents prefire and postfire displays for observing the system's performance. The debugger alternates between prefire and postfire states. In the prefire state, before the selected rule is fired, the conflict set of rules eligible to be fired, the selected rule to be fired, and the data for that rule are displayed in separate windows. Entry of the STEP command fires the rule and changes the debugger's state to postfire. In the postfire state, the just-fired rule and data updated thereby are displayed. Entry of the STEP command in the postfire state changes the state to prefire and automatically presents the prefire display for the next selected rule.
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