\left[ \begin{array}{cccc} 1 & 2 & 3 & 4 \\ 0 & 1 & 2 & 3 \\ 0 & 0 & 1 & 2 \\ 0 & 0 & 0 & 1 \end{array} \right]\cdot{\left[ \begin{array}{cccc} a & b & c & d \\ e & f & g & h \\ m & n & p & q \\ r & s & t & k \end{array} \right]}=I