1. Institute of Mathematics of the Romanian Academy, PO Box 1 – 764, 70700 Bucureşti, Romania
2. Leiden Institute of Advanced Computer Science (LIACS), Leiden University, Niels Bohrweg 1, 2333 CA Leiden, The Netherlands
3. Department of Computer Science, University of Colorado at Boulder, Boulder, CO 80309, U.S.A
4. Department of Mathematics, School of Education, Waseda University, 1-6-1 Nishi-waseda, Shinjuku-ku, Tokyo 169-8050, Japan