Computer Learning Research Center, University of London, Royal Holloway, UK and Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland;Centrum voor Wiskunde en Informatica, Amsterdam, The Netherlands;Laboratory of Computational Engineering, Helsinki University of Technology, Finland;Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland and Department of Computer Science, University of Helsinki, Helsinki, Finland;Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland and Department of Computer Science, University of Helsinki, Helsinki, Finland;Department of Computer Science, University of Helsinki, Helsinki, Finland