$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
11.3
\begin{equation} \label{diagram:homotopy-pullback-square-A} \begin{gathered} \xymatrix@R =50pt@C=50pt{ Y \ar [r] \ar [d] & X \ar [d] \\ T \ar [r] & S. } \end{gathered}\end{equation}