$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.40
\begin{equation} \label{diagram:homotopy-pullback-square5} \begin{gathered} \xymatrix@R =50pt@C=50pt{ X_{01} \ar [r] \ar [d]^{q'} & X_0 \ar [d]^{q} \\ X_1 \ar [r] & X, } \end{gathered}\end{equation}