$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.26
\begin{equation} \begin{gathered}\label{equation:dual-Beck-Chevalley-nonsense2} \xymatrix@R =50pt@C=50pt{ U(X) \ar [r]^-{ U(g') } \ar [d]^{ U(f) } & U(X_0) \ar [d]^{ \overline{f}' } \ar [d] \\ U(X_1) \ar [r]^-{ \overline{g}} & \overline{X}_{01} } \end{gathered} \end{equation}