$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
10.26
\begin{equation} \begin{gathered}\label{equation:pullback-of-quotient} \xymatrix@R =50pt@C=50pt{ X' \ar [d]^{f'} \ar [r] & X \ar [d]^{f} \\ Y' \ar [r]^-{u} & Y. } \end{gathered} \end{equation}