(Undefined (Arity Union))
(<- (Union @Sets) (Setofall ?X (Exists (?S) (And (Item ?S (Listof @Sets)) (Member ?X ?S))))) (Instance-Of (Arity Union) Undefined) (=> (Union $X $Y) (Set $Y)) (=> (= (Union @Sets) ?Set) (=> (Item ?S (Listof @Sets)) (Set ?S))) (<=> (Set-Partition ?S @Sets) (And (= ?S (Union @Sets)) (Pairwise-Disjoint @Sets))) (<=> (Set-Cover ?S @Sets) (Subset ?S (Union @Sets)))