Faculty of Environment and Information Studies, Keio University, 5322, Endo, Fujisawa City, Japan 252-8520;Technische Universität Carolo-Wilhelmina zu Braunschweig, Braunschweig, Germany 38106;Computing Department, InfoLab 2, South Drive, Lancaster University, Lancaster, UK LA1 4WA;Microsoft Research, One Microsoft Way, Redmond, USA 98052-6399;Department of Information Systems and Multimedia Design, Tokyo Denki University, Tokyo, Japan 101---8457