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