1. Department of Computer Science, University of Warwick, Warwick, Coventry, CV4 7AL, United Kingdom
2. Department of Computer Studies, Loughborough University of Technology, Loughborough, Leicestershire, LE1 3TU, United Kingdom
3. Department of Informatics, Institute of Mathematics, Slovak Academy of Sciences, P. O. Box 56, 840 00 Bratislava, Slovak Republic