$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
9.9
\begin{equation} \begin{gathered}\label{equation:square-in-simplex} \xymatrix@R =50pt@C=50pt{ \bullet \ar [r] \ar [d] \ar [dr] & \bullet \ar [d] \\ \bullet \ar [r] \ar@ {-->}[ur] & \bullet . } \end{gathered} \end{equation}