Department of Physics, Eidgenössische Technische Hochschule, CH-8049 Zurich, Switzerland
*Work partially supported by National Science Foundation Grant No. GP-31674X and by a Guggenheim Memorial Foundation Fellowship. Permanent address: Department of Mathematics, Massachusetts Institute of Technology, Cambridge, Mass. 02139.
†Alfred P. Sloan Foundation Fellow. Permanent address: Department of Mathematics and Department of Physics, Princeton University, Princeton, N.J. 08540.