Department of Computer Science, Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland. leena.salmela@cs.helsinki.fi|Department of Computer Science, Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland. leena.salmela@cs.helsinki.fi|Department of Computer Science, Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland. leena.salmela@cs.helsinki.fi|Department of Computer Science, Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland. leena.salmela@cs.helsinki.fi|Department of Computer Science, Helsinki Institute for Information Technology, University of Helsinki, Helsinki, Finland. leena.salmela@cs.helsinki.fi