\begin{bmatrix}
1 & 1 & -1 & -1 & 0 &  0 &  0 & 0 &  0 \\
0 & 0 &  0 &  1 & 1 & -1 & -1 & 0 &  0 \\
0 & 0 &  0 & -1 & 0 &  0 &  0 & 1 & -1
\end{bmatrix}