proposition: Lemma setDS A B C : A \subset B -> C :\: B \subset C :\: A.
proof: Proof. by rewrite !setDE -setCS; apply: setIS. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)