$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$
10.18
\begin{equation} \begin{gathered}\label{equation:bottom-of-Cech-nerve} \xymatrix@C =50pt@R=50pt{ X \times _{Y} X \ar@ <.4ex>[r]^-{\pi _0} \ar@ <-.4ex>[r]_-{\pi _1} & X \ar [r]^-{f} & Y. } \end{gathered} \end{equation}