This work was partly supported by ARPA (ONR) grant N00014-94-1-0775 and partly done in 1994 while the author was Meyerhoff Visiting Professor at the Weizmann Institute of Science, Rehovot, Israel.
More recently, this research has been partly supported by ARPA contract no. USC 621915, the ARPA/Rome Laboratory planning initiative under grant (ONR) N00014-94-1-0775 and ARPA/AFOSR under (AFOSR) grant # F49620-97-1-0207.
Thanks to Yoav Shoham and Aaron Sloman for email comments and to Sasa Buvac, Tom Costello and Donald Michie for face-to-face comments.
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