First Author:   Tristan-Pierre Maury MODEM, University of Paris X - Nanterre tmaury@u-paris10.fr   MODEM Bât. G, University of Paris X - Nanterre, 200, Avenue de la République Nanterre Cedex, 92001, France   33140977817(Phone) 33140977817(Fax) Second Author: Fabien Tripier MODEM, University of Paris X - Nanterre tripier@u-paris10.fr   MODEM Bât. G, University of Paris X - Nanterre, 200, Avenue de la République Nanterre Cedex, 92001, France   33140977817(Phone) 33140977784(Fax)