1. Institute for Theoretical Physics, ETH Zürich, CH-8093 Zürich, Switzerland
2. IHES, Bures-sur-Yvette, France
3. Department of Mathematics, University of Stuttgart, Pfaffenwaldring 57, D-70569, Stuttgart, Germany
4. Department of Mathematics, University of Toronto, 40, St. George Street, Bahen Centre, Toronto, ON M5S 2E4, Canada