In Prolog, define the isUnion predicate so that isUnion(X,Y,Z) says that the union of X and Y is Z. Do not use the predefined list predicates. Your predicate may choose a fixed order for Z. If you query isUnion([1,2],[3],Z) it should find a binding for Z, but it need not succeed on both isUnion([1],[2],[1,2]) and isUnion([1],[2],[2,1]). Your predicate need not work well when X or Y are unbound variables.