HIIT, Department of Computer Science, PO Box 68, FI-00014 University of Helsinki, Finland. niina.haiminen@cs.helsinki.fi|HIIT, Department of Computer Science, PO Box 68, FI-00014 University of Helsinki, Finland. niina.haiminen@cs.helsinki.fi|HIIT, Department of Computer Science, PO Box 68, FI-00014 University of Helsinki, Finland. niina.haiminen@cs.helsinki.fi