1. Interuniversity Research Centre on Enterprise Networks; Logistics and Transportation (CIRRELT); Montreal Canada
2. Department of Statistics and Operations Research; University of Vienna; Oskar-Morgenstern-Platz 1 1090 Vienna Austria
3. Department of Mathematics and Industrial Engineering; École Polytechnique de Montréal; P.O. Box 6079 Station Centre-ville Montreal Canada H3C 3A7