proposition: Lemma cprodm_norm : K \subset 'N(H).
proof: Proof. by rewrite cents_norm //; case/cprodP: eqHK_G. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)