$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
9.24
\begin{equation} \begin{gathered}\label{equation:pullback-over-discrete} \xymatrix { X_{01} \ar [r] \ar [d] & X_0 \ar [d] \\ X_1 \ar [r] & X } \end{gathered} \end{equation}