1. Department of Technology Management, Eindhoven University of Technology, P.O. Box 513 Eindhoven, NL-5600 MB, The Netherlands
2. Department of Computer Science, University of Colorado at Boulder, Campus Box 430, Boulder, CO 80309-0430, USA
3. Department of Computer Science, State University of Campinas, Caixa Postal 6176 Campinas, 13083-970, Brazil