$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.21
\begin{equation} \begin{gathered}\label{equation:categorical-pullback-square} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}_{01} \ar [r] \ar [d] & \operatorname{\mathcal{C}}_0 \ar [d] \\ \operatorname{\mathcal{C}}_1 \ar [r] & \operatorname{\mathcal{C}}. } \end{gathered} \end{equation}