1. Department of Applied Mathematics, Twente University of Technology, PO Box 217, 7500 AE Enschede, The Netherlands
2. Department of Computer Science, Aarhus University, Ny Munkegade, DK-8000 Aarhus C, Denmark and Cornell University, Ithaca, New York
3. Department of Computer Science, University of Utrecht, Budapestlaan 6, P O Box 80 012, 3508 TA Utrecht, the Netherlands and The Pennsylvania State University, University Park, Pennsylvania