Affiliation: | Department of Computer Engineering, Sharif University of Technology School of Computer Science, Institute for Research in Fundamental Sciences (IPM), Tehran, Iran. Tel.: +98 21 66164622; fax: +98 21 22828687.; Department of Computing Science, University of Glasgow, Glasgow G12 8RZ, United Kingdom. Tel.: +44 141 3304262; fax: +44 141 3304913. |