1. Laboratoire de Recherche en Informatique (LRI, UMR 8623); Université Paris Sud; 91400 Orsay France
2. Department of Computer Science; University of São Paulo; Rua do Matão, 1010; 05508-090 São Paulo Brazil
3. LIG, Grenoble University; 51, avenue Jean Kuntzmann; 38330 Montbonnot France
4. Institut universitaire de France; 103, bd Saint-Michel; 75005 Paris France