1. Department of Algebra, Faculty of Mathematics and Physics, Charles University, Sokolovská 83, 186 75 Prague 8, Czech Republic
2. Department of Mathematics, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, 166 27 Prague 6, Czech Republic
3. Research Institute for Symbolic Computation, Johannes Kepler University, Altenbergerstr, 69, A-4040 Linz, Austria