proposition: Lemma disjointWl A B C : A \subset B -> [disjoint B & C] -> [disjoint A & C].
proof: Proof. by rewrite 2!disjoint_subset; apply: subset_trans. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path div)