$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.6
\begin{equation} \begin{gathered}\label{equation:thin-in-correspondence} \xymatrix@R =50pt@C=50pt{ X_{0,0} \ar [dr] & & X_{1,1} \ar [dr] \ar [dl] & & X_{2,2} \ar [dl] \\ & X_{0,1} \ar [dr] & & X_{1,2} \ar [dl] & \\ & & X_{0,2} & & } \end{gathered} \end{equation}