1. Rényi Institute of Mathematics, Hungarian Academy of Sciences, POBox 127, 1364 Budapest, Hungary, and Department of Mathematics, University College London, Gower Street, London WC1E 6BT, England
2. Department of Applied Mathematics, Charles University, Malostranské nám. 25, 118 00 Praha 1, Czech Republic, and Department of Computer Science, ETH Zurich, 8092 Zurich, Switzerland
3. Department of Mathematics, Western Kentucky University, 1906 College Heights Blvd. #11078, Bowling Green, KY 42101, USA