proposition: Lemma joinIB x y : (x && y) || sub x y = x.
proof: Proof. by case: x y => [] []. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(path fintype tuple bigop finset div prime finfun) mathcomp(finset)