1. Rényi Institute of Mathematics; Hungarian Academy of Sciences; H-1364 Budapest Hungary
2. Department of Mathematics; University College London; London WC1E 6BT England
3. Karlsruhe Institute of Technology, Department of Mathematics; D-76128 Karlsruhe Germany
4. Department of Mathematics; University of Osnabrück; D-49076 Osnabrück Germany
5. Department of Mathematics; Albert-Ludwigs-Universität; D-79104 Freiburg i. Br. Germany