1. Information and Software Engineering Group; Vienna University of Technology, Institute of Software Technology and Interactive Systems, Favoritenstrasse 9-11/188; Vienna 1040 Austria
2. University of Toronto, Faculty of Information, 140 St George Street, Toronto, ON, M5S 3G6, Canada; and Information and Software Engineering Group, Vienna University of Technology, Institute of Software Technology and Interactive Systems, Favoritenstrasse 9-11/188; Vienna 1040 Austria