1. Department of Mathematical Sciences, Durham University, South Road, Durham DH1 3LE, UK
2. Institut de Mathématiques de Bourgogne, Université de Bourgogne, 9 av. Alain Savary, BP 47 870, 21078 Dijon, France
3. Dip. di Fisica Teorica and INFN, Università di Torino, Via P. Giuria 1, 10125 Torino, Italy