f.
move=> eqU12 eqV12 [ltUV1 maxU1].
by split=> [|W]; rewrite -(lt_eqmx eFact mx_subseries_module' V i : mx_subseries V -> mxmodule rG (0 :: V)`_i.
Proof. by move=> modV; rewrite mx_subseries_module //= mxmodule0. Qed.Definition subseries_repr V i (modV : all modG V) :=
  section_repr (mx_subseries_modu

(* Specifications: *)
Definition series_repr V i (compV : mx_composition_series V) :=
by apply: leq_trans (IHV _ ltV); rewrite addSnnS leq_add2l. Qed.
by rewrite (subseq_trans sU0U) // -defU cat_subseq // -drop_nth ?subseq_cons. Qed.