proposition: Lemma quotient_norms A B : A \subset 'N(B) -> A / H \subset 'N(B / H).
proof: Proof. exact: morphim_norms. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)