proposition: Lemma quotientInorm A : 'N_A(H) / H = A / H.
proof: Proof. by rewrite /quotient setIC morphimIdom. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)