$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.55
\begin{equation} \label{equation:mather-first-cube4} \begin{gathered} \xymatrix@R =50pt@C=50pt{ \overline{A} \ar [r] \ar [d]^{ f_{\overline{A} \overline{C}}} & \overline{B} \ar [d] & A \ar [r] \ar [d]^{ f_{AC} } & B \ar [d] \\ \overline{C} \ar [r] & \overline{D} & C \ar [r] & D } \end{gathered} \end{equation}