[1] Polish Acad Sci, Inst Matemat, Ul Sniadeckich 8, PL-00656 Warsaw, Poland
[2] Univ Copenhagen, Dept Math, Univ Pk 5, DK-2100 Copenhagen, Denmark
[3] Univ Wollongong, Sch Math & Appl Stat, Wollongong, NSW 2522, Australia
[4] Univ Lodz, Fac Phys & Appl Informat, Dept Comp Sci, Pomorska 149-153, PL-90236 Lodz, Poland