1. Department of Computer and Information Science, University of Pennsylvania, 3330 Walnut St, PA-19104 Philadelphia, USA
2. Laboratoire de Recherche en Informatique, CNRS UMR 8623, Université Paris-Sud 11, 91405 Orsay, France
3. Arizona State University, Tempe AZ, USA
4. University of Maryland, College Park MD, USA
5. Universidad Simón Bolívar, Caracas, Venezuela