[rules-users] Re-insert modified instance of same fact and Truth maintenance

Could be a basic question, but thought I should get clarified. I insert a fact say, a Computer object with some attributes ( hdd=10GB ) set, in working memory. Then I have a rule that checks if a Computer with 10GB hdd is inserted .. if yes, then it does a logicalInsert of an Alert object with computerId = this computer's id. Now I would like to re-insert the same fact with HDD changed to '20gb' and would like drools to automatically retract the Alert for this computerId .. how do I achieve this ? -abhay