$\left[\begin{array}{cc} -4 &1\cr -4 &4 \end{array}\right]$ $\left[\Rule{0pt}{2.4em}{0pt}\right.$$\left]\Rule{0pt}{2.4em}{0pt}\right.$ $= \left[\begin{array}{cc} 1 &0\cr 0 &1 \end{array}\right]$.