1. National ICT Australia, Sydney, Australia. School of Computer Science and Engineering, University of New South Wales, Sydney, Australia. rvg@cs.stanford.edu
2. Department of Mathematics and Computer Science, Technische Universiteit Eindhoven, The Netherlands. CWI, The Netherlands. s.p.luttik@tue.nl
3. Department of Mathematics and Computer Science, Technische Universiteit Eindhoven, The Netherlands. n.trcka@tue.nl