$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.42
\begin{equation} \label{diagram:homotopy-pullback-square4} \begin{gathered} \xymatrix@R =50pt@C=50pt{ X_{01} \ar [r] \ar [d] & \{ 0\} \ar [d]^{f_0} \\ \{ 1\} \ar [r]^-{f_1} & \Delta ^1 } \end{gathered}\end{equation}