$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
8.1
\begin{equation} \begin{gathered}\label{equation:twisted-arrow-canonical-isomorphism} \xymatrix@R =50pt@C=50pt{ X_0 \ar [d]^-{f_0} & X_1 \ar [l]_-{u_1} & X_2 \ar [l]_-{u_2} & \cdots \ar [l] & X_ n \ar [l]_-{u_ n} \\ Y_0 \ar [r]^-{v_1} & Y_1 \ar [r]^-{v_2} & Y_2 \ar [r] & \cdots \ar [r]^-{v_ n} & Y_ n } \end{gathered} \end{equation}