# coercion of matrix of matrices to single matrix

I have a need to construct matrices like the following:

A simple 2x2 matrix m1 = matrix([[a,b],[c,d]])

Then I need to make larger matrices out of them, as in M1 = matrix([[m1,0],[0,1]])

Big M1 has parent: mat2x2(mat2x2(SR)). However, to manipulate the matrix and do matrix x matrix multiplication I need this to be mat4x4(SR).

How can I do this?