1. Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
2. Departamento de Ciência da Computação,
IME-USP, São Paulo SP 05508-090, Brazil
and Departamento de Ciência da Computação, ICEx-UFMG, Belo Horizonte MG 31270-901,
Brazil
3. Department of Combinatorics and Optimization,
University of Waterloo, Waterloo, Ontario N2L3G1,
Canada
4. Department of Computer Science, University College London, London WC1E 6BT, United Kingdom and Institute
of Natural Sciences, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of
China