$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.56
\begin{equation} \label{equation:mather-first-cube5} \begin{gathered} \xymatrix@R =50pt@C=50pt{ \overline{C} \ar [r] \ar [d]^{ f_{\overline{C} C}} & \overline{D} \ar [d]^{ f_{ \overline{D} D}} & \overline{B} \ar [r] \ar [d]^{ f_{\overline{B} B}} & \overline{D} \ar [d]^{ f_{\overline{D} D}} \\ C \ar [r] & D & B \ar [r] & D } \end{gathered} \end{equation}