$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
7.86
\begin{equation} \begin{gathered}\label{equation:pushout-also-pullback} \xymatrix { \emptyset \ar [r] \ar [d] & X_0 \ar [d]^{f_0} \\ X_1 \ar [r]^{f_1} & X } \end{gathered} \end{equation}