proposition: Lemma quotientI A B : (A :&: B) / H \subset A / H :&: B / H.
proof: Proof. exact: morphimI. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)