$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.64
\begin{equation} \begin{gathered}\label{equation:covariant-transport-categorical-pullback-square4} \xymatrix@R =50pt@C=50pt{ \tau _0 \ar [r] \ar [d] & \tau \ar [d] \\ \underline{\Delta ^0} \ar [r] & \sigma } \end{gathered} \end{equation}