proposition: Lemma setUSS A B C D : A \subset C -> B \subset D -> A :|: B \subset C :|: D.
proof: Proof. by move=> /(setSU B) /subset_trans sAC /(setUS C)/sAC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)