proposition: Lemma classes_quotient A : A \subset 'N(H) -> classes (A / H) = [set xA / H | xA in classes A].
proof: Proof. exact: classes_morphim. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)