$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
10.28
\begin{equation} \begin{gathered}\label{equation:regular-via-universal-quotient2} \xymatrix@C =50pt@R=50pt{ Y' \times _{Y} X \ar [r]^-{q'} \ar [d] & Y' \times _{Y} Y_0 \ar [r]^-{i'} \ar [d] & Y' \ar [d]^{u} \\ X \ar [r]^-{q} & Y_0 \ar [r]^-{i} & Y, } \end{gathered} \end{equation}