proposition: Lemma quotient_homg A : A \subset 'N(H) -> homg (A / H) A.
proof: Proof. exact: morphim_homg. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)