$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.27
\begin{equation} \begin{gathered}\label{equation:categorical-pullback-square4} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}' \ar [r] \ar [d] & \operatorname{\mathcal{C}}\ar [d]^{U} \\ \operatorname{\mathcal{D}}' \ar [r] & \operatorname{\mathcal{D}}, } \end{gathered} \end{equation}