% Acknowledgments

\section{Acknowledgments}

The authors would like to thank Zena Ariola, James Hicks, Jan-Willem
Maessen, and Rishiyur Nikhil for several insightful discussions that
immensely helped in distilling the important issues while formalizing
the semantics of barriers.

The research described in this paper was funded in part by the
Advanced Research Projects Agency of the Department of Defense under
Office of Naval Research contract N00014-92-J-1310.


