proposition: Lemma setSU A B C : A \subset B -> A :|: C \subset B :|: C.
proof: Proof. by move=> sAB; rewrite -!(setUC C) setUS. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)