proposition: Lemma sub_lcosetV x A B : (A \subset x^-1 *: B) = (x *: A \subset B).
proof: Proof. by rewrite sub_lcoset invgK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)