1. Department of Computer Science, Universidade Federal de Minas Gerais, Belo Horizonte, MG 31270-901, Brazil.
2. Department of Combinatorics and Optimization, University of Waterloo, Waterloo, ON N2L 3G1, Canada.
3. Institut für Theoretische Physik, Leibniz Universität Hannover, Hannover, Germany.