$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.56
\begin{equation} \begin{gathered}\label{equation:categorical-pullback-square-slice2} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}'_{/q} \ar [r] \ar [d] & \operatorname{\mathcal{C}}_{ / (G \circ q)} \ar [d] \\ \operatorname{\mathcal{D}}'_{ / (F' \circ q)} \ar [r] & \operatorname{\mathcal{D}}_{ / (F \circ G \circ q)}. } \end{gathered} \end{equation}