$$\left(\begin{array}{cc}\left(\begin{array}{cccc}{a}_1& a_2& a_3& a_4\\ a_5& a_6& a_7& a_8\end{array}\right)& \left(\begin{array}{c}{b}_1\\ b_2\\ b_3\\ b_4\end{array}\right)\\ \begin{array}{cc}\mathrm{\hspace{0.5em}<!--\; \hspace{0.5em}\; -->}\mathrm{\hspace{1em}<!--\; \hspace{1em}\; -->}0\mathrm{\hspace{0.33em}<!--\; \hspace{0.33em}\; -->}& \left(\begin{array}{cc}{c}_1& c_2\\ c_3& c_4\end{array}\right)\end{array}& \end{array}\right)$$