Department of Computer Science, The University of Texas at Austin, Austin, TX 78712, USA, Departement dinformatique, Ecole Normale Superieure, 45 Rue dUlm, F-75230 Paris Cedex 05, France and Department of Electrical Engineering, The University of Southern California, Los Angeles, CA 90089, USA Department of Computer Science, The University of Texas at Austin, Austin, TX 78712, USA, Departement dinformatique, Ecole Normale Superieure, 45 Rue dUlm, F-75230 Paris Cedex 05, France and Department of Electrical Engineering, The University of Southern California, Los Angeles, CA 90089, USA.