proposition: Lemma quotientYK G : G \subset 'N(H) -> coset H @*^-1 (G / H) = H <*> G.
proof: Proof. by move=> nHG; rewrite quotientK ?norm_joinEr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)