\documentclass{ppex}
\nonstopmode
\usepackage [graph,curve]{xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [graph,curve]{xy}
\[\xygraph{
[]L :@/_/ [r]{M_1} :@/^/ [r]{M^2}
    :@{|.>}[r]R    :@/_1em/"L"
}\]
\end{document}
