Institution: | a Department of Mathematics and Computer Science, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands b CWI, P.O. Box 94079, 1090 GB Amsterdam, The Netherlands c Bell Laboratories, Lucent Technologies, P.O. Box 636, Murray Hill, NJ 07974, USA d Mathematical Institute, University of Wroc?aw, pl. Grunwaldzki 2/4, 50-384 Wroc?aw, Poland |