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