$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
7.51
\begin{equation} \begin{gathered}\label{equation:homotopy-pullback-as-homotopy-limit} \xymatrix@R =50pt@C=50pt{ X_{01} \ar [r] \ar [d] & X_{0} \ar [d] \\ X_{1} \ar [r] & X, } \end{gathered} \end{equation}