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