I have several Prolog facts indicating that something or someone is either a person, location or object. I have a clause go(person,location) that indicated that a person moves from where they are to the location given in the clause. However, when I ask the relevant query to find out if someone is at a certain location, Prolog responds with every person that was ever there according to the clauses. How do I go about writing a rule that says that if you are in one location you are by definition not in any of the others?