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