A preliminary version of this article was accepted for the 1998 NCAI but was withdrawn in disagreement with AAAI's policy of requiring assignment of copyright. 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.