1. a Faculty of Informatics , University of Debrecen , H-4010 , Debrecen , P.O. Box 12, Hungary
2. b Bolyai Institute, University of Szeged , H-6720 , Szeged , Aradi vértanúk tere 1, Hungary
3. c Institute for Mathematics, Astrophysics and Particle Physics, Radboud University Nijmegen , Toernooiveld 1, 6525 ED , Nijmegen , The Netherlands