$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.79
\begin{equation} \begin{gathered}\label{equation:homotopy-pullback-square-truncatedness} \xymatrix@R =50pt@C=50pt{ X \ar [r] \ar [d]^{f} & X' \ar [d]^{f'} \\ Y \ar [r]^-{g} & Y'. } \end{gathered} \end{equation}