1. Institute of Discrete Mathematics and Geometry, Vienna University of Technology, Wiedner Hauptstraße 8–10, 1040 Vienna, Austria
2. Department of Mathematics, University of Ghent, Krijgslaan 281-S8, B9000 Ghent, Belgium
3. Division of Mathematical Logic, Steklov Mathematical Institute of the Russian Academy of Sciences, Ulitsa Gubkina 8, 117966 Moscow, Russia