1. Solmaria Halleck Vega, Paris School of Economics, Centre d'Economie de la Sorbonne, Université Paris 1 Panthéon-Sorbonne, 106-112 Blvd. de l'Hôpital, Paris Cedex 13 F-75647, France
2. J. Paul Elhorst, Faculty of Economics and Business, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands