First Author:   François Seck Fall Lereps (University of Toulouse), CRES (University of Dakar) francois-seck.fall@ut-capitole.fr   Toulouse Cedex, 31042, France   (+33) 5 61 12 87 07(Phone) (+33) 5 61 12 87 07(Fax) http://appliphp.univ-tlse1.fr/LEREPS/spip/