(Undefined (Arity Difference)) (Nth-Domain Difference 3 Set) (Nth-Domain Difference 1 Set) (= (Difference ?Set @Sets) (Setofall ?X (And (Member ?X ?Set) (Forall (?S) (=> (Item ?S (Listof @Sets)) (Not (Member ?X ?S))))))) (=> (= (Difference ?Set @Sets) ?Diff-Set) (=> (Item ?S (Listof @Sets)) (Set ?S)))