Section DsumRepr.
Variables (n : nat) (rG : reprG n).

Lemma mx_rsim_dsum (I : finType) (P : pred I) U rU (W : 'M_n)
(modU : forall i, mxmodule rG (U i)) (modW : mxmodule rG W) :
let S := (\sum_(i | P i) U i)%MS in (S :=: W)%MS -> mxdirect S ->
(forall i, mx_rsim (submod_repr (modU i)) (rU i : representation)) ->
mx_rsim (submod_repr modW) (\big[dadd_grepr/grepr0]_(i | P i) rU i).