Two sets are disjoint if and only if there is no object that is a member of both sets.
(<=> (Disjoint ?S1 ?S2) (Empty (Intersection ?S1 ?S2)))
(<=> (Disjoint ?S1 ?S2) (Empty (Intersection ?S1 ?S2))) (Forall (?S1 ?S2) (=> (Item ?S1 (Listof @Sets)) (Item ?S2 (Listof @Sets)) (Or (= ?S1 ?S2) (Disjoint ?S1 ?S2)))) (<=> (Pairwise-Disjoint @Sets) (Forall (?S1 ?S2) (=> (Item ?S1 (Listof @Sets)) (Item ?S2 (Listof @Sets)) (Or (= ?S1 ?S2) (Disjoint ?S1 ?S2))))) (Forall (?S) (=> (Not (Empty ?S)) (Exists (?U) (And (Member ?U ?S) (Disjoint ?U ?S)))))