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