1. School of Mathematical Sciences Queen Mary and Westfield College, Mile End Road London El 4NS
2. Department of Computing Imperial College, 180 Queen's Gate London SW7 2BZ
3. CNRS Université Paris 7 Equipe de Logique Mathématique, Tour 45–55, 5ème étage 2 Place Jussieu 75251 Paris Cedex 05 France
4. Department of Mathematics Hebrew University, Jerusalem 91904 Israel