proposition: Lemma quotientR A B : A \subset 'N(H) -> B \subset 'N(H) -> [~: A, B] / H = [~: A / H, B / H].
proof: Proof. exact: morphimR. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)