$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.61
\begin{equation} \begin{gathered}\label{equation:characterize-initial-by-slice} \xymatrix@R =50pt@C=50pt{ \operatorname{\partial \Delta }^{n-1} \ar [r] \ar [d] & \operatorname{\mathcal{C}}_{Y/} \ar [d] \\ \Delta ^{n-1} \ar@ {-->}[ur] \ar [r] & \operatorname{\mathcal{C}}, } \end{gathered} \end{equation}