(* For us, "semisimple" means "sum of simple modules"; this is classically,   *)
(* but not intuitionistically, equivalent to the "completely reducible"       *)
(* alternate characterization.                                                *)
Implicit Type I : finType.
Variant mxsemisimple (V : 'M_n) :=
MxSemisimple I U (W := (\sum_(i : I) U i)%MS) of
forall i, mxsimple (U i) & (W :=: V)%MS & mxdirect W.

Lemma hom_mxsemisimple (V f : 'M_n) :
mxsemisimple V -> (V <= dom_hom_mx f)%MS -> mxsemisimple (V *m f).