$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.29
\begin{equation} \begin{gathered}\label{equation:general-categorical-pullback} \xymatrix@R =50pt@C=50pt{ X_{01} \ar [r] \ar [d] & X_0 \ar [d] \\ X_{1} \ar [r] & X. } \end{gathered} \end{equation}