Section BlockMatrixRing.
Context {R : ringType} {p q : nat} {p_ : 'I_p -> nat} {q_ : 'I_q -> nat}.
Notation sp := (\sum_i p_ i)%N.
Notation sq := (\sum_i q_ i)%N.

Lemma mul_mxrow_mxblock m
(R_ : forall i, 'M[R]_(m, p_ i)) (B_ : forall i j, 'M[R]_(p_ i, q_ j)) :
\mxrow_i R_ i *m \mxblock_(i, j) B_ i j = \mxrow_j (\sum_i (R_ i *m B_ i j)).