Helsinki Institute for Information Technology HIIT, Department of Information and Computer Science, Aalto University School of Science and Technology, Aalto, Finland. leo.lahti@iki.fi|Helsinki Institute for Information Technology HIIT, Department of Information and Computer Science, Aalto University School of Science and Technology, Aalto, Finland. leo.lahti@iki.fi|Helsinki Institute for Information Technology HIIT, Department of Information and Computer Science, Aalto University School of Science and Technology, Aalto, Finland. leo.lahti@iki.fi