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

Lemma submxcolD m (B B' : 'M[V]_(sp, m)) i :
submxcol (B + B') i = submxcol B i + submxcol B' i.