1. Department of Stochastics, Institute of Mathematics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary. Alfréd Rényi Institute of Mathematics, Reáltanoda utca 13-15, 1053 Budapest, Hungary
2. The Czech Academy of Sciences, Institute of Information Theory and Automation, Pod vodárenskou věží 4, 18200 Praha 8, Czech Republic
3. Department of Stochastics, Institute of Mathematics, Budapest University of Technology and Műegyetem rkp. 3., H-1111 Budapest, Hungary. Alfréd Rényi Institute of Mathematics, Reáltanoda utca 13-15, 1053 Budapest, Hungary