$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
9.11
\begin{equation} \begin{gathered}\label{equation:lifting-problem-in-homotopy} \xymatrix@R =50pt@C=50pt{ A \ar [d]^{f} \ar [r]^-{u_0} & X \ar [d]^{g} \\ B \ar [r]^-{ \overline{u} } \ar@ {-->}[ur] & Y } \end{gathered} \end{equation}