(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)))))))
(Instance-Of (Arity Difference) Undefined)
(=> (= (Difference ?Set @Sets) ?Diff-Set)
(=> (Item ?S (Listof @Sets)) (Set ?S)))