1. Siemens Electronic Design Automation, Wilsonville, OR 97070 USA.
2. School of Computer Science, University of Sydney, Sydney, NSW 2006, Australia.
3. Department of Mathematics and Computer Science, TU Eindhoven, Eindhoven, 5612 AZ, The Netherlands.
4. Department of Information and Computing Sciences, Utrecht University, Utrecht, 3584 CS, The Netherlands.