proposition: Lemma card_quotient A : A \subset 'N(H) -> #|A / H| = #|A : H|.
proof: Proof. by move=> nHA; rewrite card_quotient_subnorm (setIidPl nHA). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)