$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.61
\begin{equation} \begin{gathered}\label{equation:covariant-transport-categorical-pullback-square} \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}_{00} \ar [dr]^{ U_{00} } \ar [rr] \ar [dd] & & \operatorname{\mathcal{E}}_{01} \ar [dd] \ar [dr]^{U_{01}} & \\ & \operatorname{\mathcal{D}}_{00} \ar [rr] \ar [dd] & & \operatorname{\mathcal{D}}_{01} \ar [dd] \\ \operatorname{\mathcal{E}}_{10} \ar [dr]^{U_{10} } \ar [rr] & & \operatorname{\mathcal{E}}_{11} \ar [dr]^{U_{11} } & \\ & \operatorname{\mathcal{D}}_{10} \ar [rr] & & \operatorname{\mathcal{D}}_{11} } \end{gathered} \end{equation}