1. Institute of Computer Science, Jagiellonian University, Łojasiewicza 6, 30-348 Kraków, Poland. forysw@ii.uj.edu.pl
2. Departamento de Matemáticas, Universidad de Murcia, Campus de Espinardo, 30100 Murcia, Spain
3. Faculty of Applied Mathematics, AGH University of Science and Technology, al.Mickiewicza 30, 30-059 Kraków, Poland. oprocha@agh.edu.pl