$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.26
\begin{equation} \begin{gathered}\label{equation:categorical-pullback-square3} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}' \ar [r] \ar [d]^{F'} & \operatorname{\mathcal{C}}\ar [d]^{F} \\ \operatorname{\mathcal{D}}' \ar [r] & \operatorname{\mathcal{D}}. } \end{gathered} \end{equation}