$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.7
\begin{equation} \begin{gathered}\label{equation:thin-filler-in-Corr} \xymatrix@C =50pt@R=50pt{ X_{0,0} \ar [dr] & & X_{1,1} \ar [dl]^{u} \ar [dr]_{v} & & X_{2,2} \ar [dl] \\ & X_{0,1} & & X_{1,2} & } \end{gathered} \end{equation}