1. Department of Engineering Mathematics, University of Bristol, Queen's Building, Bristol BS8 1TR, UK
2. Department of Computer Science, Concordia University, 1455 Boulevard de Maisonneuve O., Montréal Québec, H3G 1M8, Canada
3. IBM Research, PO Box 218, Yorktown Heights, NY 10598, USA
4. Department of Mathematics, Cornell University, Malott Hall, Ithaca, NY 14853–4201, USA
5. Institute for Mathematics, University of Paderborn, D-33095 Paderborn, Germany