Putting together a larger matrix from smaller matrices

[Matjaz Bezovnik]

Dear all,

I'm but a layman so do not take offence at this maybe over simple
question.

This is something which is done often in FEM methods, and the alike.

I have matrix A of 3x3 elements, and B, of the same number of
elements, 3x3. 

What would be the most obvious way to assemble a matrix which:
a11 a12 a13
a21 a22 a23
a31 a32 a33+b11 b12 b13
                      b21 b22 b23
                      b31 b32 b33

(the missing elements = zero)

(you see the pattern - if there was a third matrix C, it would
"connect" to b33 on the main diagonal)


Matjaz