1. Research Institute for Mathematical Sciences, Kyoto University, Kyoto 606, Japan
2. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
3. Institute of Mathematics, University of Augsburg, 86135 Augsburg, Germany
4. School of Business and Economics, RWTH Aachen University, 52072 Aachen, Germany
5. Department of Mathematics, ETH Zürich, 8092 Zürich, Switzerland; and Department of Applied Mathematics and Statistics, Johns Hopkins University, Baltimore, Maryland 21218