1. Department of Mathematics and Computer Science, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
2. Department of Mathematics and Computer Science, Wesleyan University Middletown, CT 06459-0128, U.S.A. , and Philips Research Laboratories, Building WO 1.075, 5656 AE Eindhoven, The Netherlands