Rotate it!

$ \left[ \begin{array}{c} \cos 90 \quad \sin 90 \\ -\sin 90 \quad \cos 90 \end{array} \right] \left[ \begin{array}{c} a_1 \\ a_2 \end{array} \right] = $

$ \left[ \begin{array}{c} a_1 \\ a_2 \end{array} \right] $

(And yes, this was just to experiment with CSS rotations!)