$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
7.31
\begin{equation} \begin{gathered}\label{equation:characterize-relative-Kan} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}^{0} \ar [r]^-{F_0} \ar [d] & \operatorname{\mathcal{D}}\ar [d]^{U} \\ \operatorname{\mathcal{C}}\ar [r]^-{ \overline{F} } \ar@ {-->}[ur] & \operatorname{\mathcal{E}}} \end{gathered} \end{equation}