$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.55
\begin{equation} \label{equation:mather-first-cube3} \begin{gathered} \xymatrix@R =50pt@C=50pt{ \overline{A} \ar [r]^-{ f_{ \overline{A} \overline{B} }} \ar [d]^{ f_{\overline{A} A}} & \overline{B} \ar [d]^{ f_{ \overline{B} B} } \\ A \ar [r]^-{f_{AB}} & B } \end{gathered} \end{equation}