proposition: Lemma central_factor_central H K : central_factor G H K -> (K / H) \subset 'Z(G / H).
proof: Proof. by case/and3P=> /quotient_cents2r *; rewrite subsetI quotientS. Qed.
imports: mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(fintype bigop finset fingroup morphism) mathcomp(automorphism quotient action commutator center)