1. MTA-BME Stochastics Research Group, Department of Stochastics, Budapest University of Technology and Economics, P.O. Box 91, 1521 Budapest, Hungary
2. School of Mathematics, University of Bristol, Bristol BS8 1SN, UK
3. Department of Physics and Mathematics, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland
4. Institute of Mathematics, Polish Academy of Sciences, Sniadeckich 8, 00-656 Warsaw, Poland