1. School of Mathematical Sciences/Institute of Computational Science; University of Electronic Science and Technology of China; Chengdu, Sichuan 611731 China
2. Johann Bernoulli Institute for Mathematics and Computer Science; University of Groningen; Nijenborgh 9 Groningen, PO Box 407 9747 AG The Netherlands
3. Faculty of Computer Science; Free University of Bozen-Bolzano; piazza Domenicani 3 Bozen-Bolzano, 39100 Italy