$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.9
\begin{equation} \begin{gathered}\label{equation:relative-outer-horn-filling} \xymatrix@R =50pt@C=50pt{ \Lambda ^{n}_{0} \ar [r]^-{ \sigma _0 } \ar [d] & \operatorname{Cospan}(\operatorname{\mathcal{C}}) \ar [d]^{ \operatorname{Cospan}(U) } \\ \Delta ^ n \ar [r]^-{ \overline{\sigma } } \ar@ {-->}[ur] & \operatorname{Cospan}(\operatorname{\mathcal{D}}), } \end{gathered} \end{equation}