proposition: Lemma quotientYidr G : G \subset 'N(H) -> G <*> H / H = G / H.
proof: Proof. move=> nHG; rewrite -genM_join quotient_gen ?mul_subG ?normG //. by rewrite quotientMidr genGid. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)