1. Faculty of Computer Science and Mathematics, University of Passau, Germany. E-mails: Jelena.Mitrovic@uni-passau.de, Siegfried.Handschuh@uni-passau.de
2. Anglia Ruskin University, UK. E-mail: cliff.oreilly@anglia.ac.uk
3. Faculty of Mathematics, University of Belgrade, Serbia. E-mail: ml.miljana@gmail.com