1. School of Mathematics Institute for Advanced Study Princeton NJ 08540 USA
2. Universität Bonn Mathematical Institute Endenicher Allee 60, D‐53115 Bonn Germany
3. Instytut Matematyczny Uniwersytet Wrocławski Plac Grunwaldzki 2/4 50‐384 Wrocław Poland
4. Department of Mathematics University of California Los Angeles CA 90095‐1555 USA