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The research reported here was supported in part by a Fulbright/Ministerio of Educación y Ciencia of Spain scholarship, in part by the Defense Advanced Research Projects Agency (DARPA) and Air Force Research Laboratory, Air Force Materiel Command, USAF, under agreement number F30602-00-1-0504, in part by the National Science Foundation under grant number IRI-9313993, in part by the Rome Laboratory of the Air Force Systems Command and the Defense Advanced Research Projects Agency (DARPA) under contract numbers F30602-94-C-0210, F30602-97-2-0352, F30602-97-2-0238, F30602-98-2-0109, in part by the United States Air Force under contract number F49620-98-1-0046, and in part by the Integrated Media Systems Center, a National Science Foundation Engineering Research Center, Cooperative Agreement No. EEC-9529152. The U.S.Government is authorized to reproduce and distribute reports for Governmental purposes notwithstanding any copyright annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of any of the above organizations or any person connected with them.