Department of Computer Science, University of Helsinki, University of Helsinki, Finland. merja.oja@tkk.fi|Department of Computer Science, University of Helsinki, University of Helsinki, Finland. merja.oja@tkk.fi|Department of Computer Science, University of Helsinki, University of Helsinki, Finland. merja.oja@tkk.fi|Department of Computer Science, University of Helsinki, University of Helsinki, Finland. merja.oja@tkk.fi