1. Departamento de Matemática, Universidade Federal do Paraná, C.P. 19096, CEP 81531-990 Curitiba, Brazil
2. Logic and Discrete Mathematics, Ghent University, Krijgslaan 281 Department of Mathematics: Analysis, , Building S8, B 9000 Ghent, Belgium
3. School of Mathematical Sciences, Queen Mary University of London , Mile End Road, London E1 4NS, United Kingdom