\documentclass{ppex}
\nonstopmode
\usepackage [frame,arrow,curve]{xy}
\pagestyle{empty}
\setlength\textwidth{183.0pt}
\begin{document}
\ResetPreambleCommands
\ReadyForTheFray
\usepackage [frame,arrow,curve]{xy}
\[\begin{xy}
  (0,0)*@{*}*[F]{Box1}!UL="a",*@{x}
 ,(20,0)*@{*}*[F]{Box2}!RD*@{x}
 ,\ar@/^20pt/"a", \POS"a"!DR(.8)*@{+}
\end{xy}\]
\end{document}