$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
10.27
\begin{equation} \begin{gathered}\label{equation:regular-via-universal-quotient} \xymatrix@C =50pt@R=50pt{ X' \ar [r]^-{f'} \ar [d] & Y' \ar [d]^{u} \\ X \ar [r]^-{f} & Y, } \end{gathered} \end{equation}