Department of Computer Science, University of Helsinki, Finland. ari.rantanen@iki.fi|Department of Computer Science, University of Helsinki, Finland. ari.rantanen@iki.fi|Department of Computer Science, University of Helsinki, Finland. ari.rantanen@iki.fi|Department of Computer Science, University of Helsinki, Finland. ari.rantanen@iki.fi|Department of Computer Science, University of Helsinki, Finland. ari.rantanen@iki.fi|Department of Computer Science, University of Helsinki, Finland. ari.rantanen@iki.fi