$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
9.4
\begin{equation} \begin{gathered}\label{equation:snoops} \xymatrix@R =50pt@C=50pt{ X \ar [r]^{f_0} \ar [d]^{f_1} & X_0 \ar [d] \\ X_1 \ar [r] & X_{01}. } \end{gathered} \end{equation}