proposition: Lemma quotientD1 A : (A / H)^# \subset A^# / H.
proof: Proof. exact: morphimD1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)