proposition: Lemma properD A B C : ssub A (B -s C) -> ssub A B /\ disjoint A C.
proof: Proof. move => [pa pb]; move/subCset_P3: pa => [pc pd]; split => //; split => //. by dneg ab; rewrite -ab; symmetry; apply/setCId_Pl. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)