This work was partly supported by ARPA (ONR) grant N00014-94-1-0775 and partly done while the author was Meyerhoff Visiting Professor at the Weizmann Institute of Science, Rehovot, Israel.
Thanks to Yoav Shoham and Aaron Sloman for email comments and to Sasa Buvac and Tom Costello for face-to-face comments.
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