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