$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.60
\begin{equation} \begin{gathered}\label{equation:Hom-in-oriented-fiber-product-reformulated} \xymatrix@R =50pt@C=50pt{ \operatorname{Tw}( \operatorname{\mathcal{C}}_{\pm } ) \ar [r]^-{ Q_{+} } \ar [d]^{Q_{-}} & \operatorname{\mathcal{E}}_{+} \ar [d]^{ \pi _{+} } \\ \operatorname{\mathcal{E}}_{-} \ar [r]^-{ \pi _{-} } & \operatorname{Tw}(\operatorname{\mathcal{C}}) } \end{gathered} \end{equation}