1. Department of Statistics, Harvard University, MA 02138, USA
2. School of Mathematics, University of Bristol, BS8 1TW, UK
3. Centre de Recherche en Mathématiques de la Décision (CEREMADE), Université Paris-Dauphine, Paris Sciences et Lettres Research University, France, and Department of Statistics, University of Warwick, 75775 PARIS CEDEX 16, UK