Sets are pairwise-disjoint if and only if every set is disjoint from every other set.
(Undefined (Arity Pairwise-Disjoint)) (<=> (Pairwise-Disjoint @Sets) (Forall (?S1 ?S2) (=> (Item ?S1 (Listof @Sets)) (Item ?S2 (Listof @Sets)) (Or (= ?S1 ?S2) (Disjoint ?S1 ?S2)))))