\documentclass{ppex}
\nonstopmode
\usepackage [curve]{xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [curve]{xy}
\[\begin{xy}
  0*+{L};(50,20)*+{R}
  **\crv{~*=<2mm>{.}(20,30)&(30,35)}
  **\crv~Lc{~**@{==}~*{\bullet}%
       (20,-15)&(30,-15)}
\end{xy}\]

\end{document}
