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* Work supported in part by the NSF grants IRI-8903507 sad CCR-9102159. On sabbatical leave from Stony Brook University, Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and tha title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission.