1. SAP AG, D-69190 Walldorf, Germany. The work of this author was done while at the Faculty of Computer Science, Electrical Engineering and Mathematics, University of Paderborn, 33102 Paderborn, Germany.
2. Department of Computer Science, University of Cyprus, Nicosia, 1678, Cyprus. Part of the work of this author was done while visiting the Faculty of Computer Science, Electrical Engineering and Mathematics, University of Paderborn.
3. Faculty of Computer Science, Electrical Engineering and Mathematics, University of Paderborn, 33102, Paderborn, Germany. Part of the work of this author was done while visiting the Department of Computer Science, University of Cyprus.
4. Department of Computer Science, University of Liverpool, L69 3BX, UK, and Research Academic Computer Technology Institute, 261 10 Patras, Greece.
5. Institute of Mathematics, Slovak Academy of Sciences, 841 04 Bratislava 4, Slovak Republic.