$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
4.77
\begin{equation} \begin{gathered}\label{equation:uniqueness-of-minimal} \xymatrix@R =50pt@C=50pt{ \sigma _0 \ar [dr]_{\alpha (\tau _0)} \ar [rr]^-{ \operatorname{id}} & & \sigma _0 \\ & \tau _0 \ar [ur]_{\beta (\tau _0)} & } \end{gathered} \end{equation}