1. Graduate School of Mathematics, Nagoya University, Japan
2. California Institute of Technology, Pasadena, Calif., U.S.A.; Department of Physics, Graduate School of Science, Nagoya University, Japan
3. California Institute of Technology, Pasadena, Calif., U.S.A.; Max-Planck-Institut für Mathematik, Bonn, Germany
4. California Institute of Technology, Pasadena, Calif., U.S.A.; Institute for Theoretical Physics, University of Amsterdam, The Netherlands; Faculty of Physics, University of Warsaw, Poland