$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
3.44
\begin{equation} \begin{gathered}\label{equation:homotopy-pullback-contractible-corner} \xymatrix@R =50pt@C=50pt{ X_{01} \ar [r] \ar [d] & X_0 \ar [d]^{f_0} \\ X_1 \ar [r]^-{f_1} & X } \end{gathered} \end{equation}