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Possible Duplicate:
Highlight elements in the matrix

i have a matrix:

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

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