1. Department of Applied Mathematics and Theoretical Physics, University of Cambridge, United Kingdom
2. Department of Mathematics, Rutgers University, Piscataway, New Jersey, U.S.A.
3. Institut des Hautes Etudes Scientifiques, Bures-sur-Yvette, France; Institut de Physique Théorique, CEA, IPhT, Gif-sur-Yvette, France; and CNRS, URA 2306, Gif-sur-Yvette, France