Definition val_factmod m : _ -> 'M_(m, n) :=
mulmxr (row_base (cokermx U) *m row_ebase U).
Definition in_factmod m : 'M_(m, n) -> _ := mulmxr (col_base (cokermx U)).
Canonical val_factmod_linear m := [linear of @val_factmod m].
Canonical in_factmod_linear m := [linear of @in_factmod m].

Lemma in_factmod_addsK m (W : 'M_(m, n)) :
(in_factmod (U + W)%MS :=: in_factmod W)%MS.