$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.23
\begin{equation} \begin{gathered}\label{equation:categorical-pullback-square55} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}_{01} \ar [r] \ar [d] & \operatorname{\mathcal{C}}_0 \ar [d]^{q} \\ \operatorname{\mathcal{C}}_1 \ar [r] & \operatorname{\mathcal{C}}, } \end{gathered} \end{equation}