$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.39
\begin{equation} \label{diagram:homotopy-pullback-square2} \begin{gathered} \xymatrix@R =50pt@C=50pt{ X' \ar [r] \ar [d]^{q'} & X \ar [d]^{q} \\ S' \ar [r] & S, } \end{gathered}\end{equation}