1. Faculty of Mathematics and Computer Science, FernUniversität in Hagen, Universitätsstr. 1, 58084 Hagen, Germany
2. Interactive Systems, Department of Computer Science and Applied Cognitive Science, University of Duisburg-Essen, 47057 Duisburg, Germany
3. Systems Engineering Section, Faculty of Technology, Policy and Management, Delft University of Technology, PO box 5015, 2600 GA Delft, Netherlands