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.