# Drawing a box around certain parts of a matrix [duplicate]

Possible Duplicate:
Highlight elements in the matrix

i have a matrix:

$$\begin{bmatrix} 1 & 5 & 4 & 2 & 1 \\ 1 & 5 & 4 & 2 & 1 \\ 1 & 5 & 4 & 2 & 1 \\ \end{bmatrix} \label{e:crop1}$$


and i would like to draw a box around a few of the values to highlight a selection & label it, how would i go about this?

I've looked at nodes but havent got a clue. thanks