proposition: Lemma divg_normal G : H <| G -> #|G| %/ #|H| = #|G / H|.
proof: Proof. by case/andP=> sHG nHG; rewrite divgS ?card_quotient. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)