### Is it possible to join matrices in to a bigger matrix?

I have two symbolic $2 \times 2$ matrices $A$ and $B$. I want to join them into a $4 \times 4$ matrix of the form
$\begin{pmatrix}A & B \cr B & A\end{pmatrix}$.

The problem is that when I give

```
C=matrix(SR,4,4)
C=matrix([[A,B],[B,A]])
```

I get a three dimensional object, not a four by four matrix.
The actual matrices I am working with are bigger than this, so I don't want to do it by hand. Is there a way of doing this without a bunch of for loops?