\documentclass{ppex}
\nonstopmode
\usepackage [all,2cell]{xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [all,2cell]{xy}
\UseAllTwocells
\[
\renewcommand{\objectstyle}{\scriptstyle}
\renewcommand{\labelstyle}{\scriptstyle}
\xymatrix @=1pc {
&& \bullet \ar[2,2]
\\ \\
   \bullet \ar[-2,2]
        \rrtwocell {=}
        \xtwocell[0,4]{}\omit{^<-4>a}
&&  x_b \ar[rr] \ar[dr]_{\mathrm{id}_x}
        \rrtwocell\omit{<1.5>}
&& \bullet
\\ &&& x_e \ar[ur]
}\]
\end{document}