$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
10.20
\begin{equation} \begin{gathered}\label{equation:sieve-in-homotopy} \xymatrix@R =50pt@C=50pt{ X' \ar [dr]_{ [f'] } \ar [rr] & & X \ar [dl]^{ [f] } \\ & Y & } \end{gathered} \end{equation}