Section BlockRowRing.
Context {R : ringType} {n : nat} {q_ : 'I_n -> nat}.
Notation sq := (\sum_i q_ i)%N.
Implicit Type (s : 'I_sq).

Lemma mul_mxrow m n' (A : 'M[R]_(m, n')) (R_ : forall j, 'M[R]_(n', q_ j)) :
A *m \mxrow_j R_ j= \mxrow_j (A *m R_ j).