$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
9.27
\begin{equation} \begin{gathered}\label{equation:pullback-of-monomorphism} \xymatrix@R =50pt@C=50pt{ X_0 \ar [d]^{i} \ar [r] & Y_0 \ar [d]^{j} \\ X \ar [r]^-{f} & Y, } \end{gathered} \end{equation}