abstract Niels Landwehr1 , Taneli Mielik¨ainen2 , Lauri Eronen2 , Hannu Toivonen1,2 , and Heikki Mannila2 1 Machine Learning Lab, Dept. of Comp. Science, University of Freiburg, Germany 2 HIIT Basic Research Unit, Dept. of Comp. Science, University of Helsinki, Finland