Section SquareBlockMatrixRing.
Context {R : ringType} {p : nat} {p_ : 'I_p -> nat}.
Notation sp := (\sum_i p_ i)%N.
Implicit Type (s : 'I_sp).

Lemma mxdiagZ a : \mxdiag_i (a%:M : 'M[R]_(p_ i)) = a%:M.