proposition: Lemma mem_remgr K B x : x \in K * B -> remgr K B x \in B.
proof: Proof. by rewrite -remgrP => /setIP[]. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)