sub_trans // morphim1.
  by rewrite morphim_lcn // (lcn_nil_classP _ _ (leqnn

(* Specifications: *)
have [sZG nZG] := andP (ucn_normal n G).
by rewrite (subset_trans _ (ucn_comm n G)) ?commSg ?subsetIr. Qed.
suffices: 'L_2(G) \subset G :&: 'L_3(G) by move/(e
End QuotientNil.
Lemma solvableS G H : H \subset G -> solvable G -> solvable H.
by rewrite -(injmSK injf) ?gFsub_trans ?morphim_der // G
Proof. exact: morphim_sol. Qed.