Max Planck Institute for Computer Science, Saarbrucken, Germany. bozek@mpi-inf.mpg.de|Max Planck Institute for Computer Science, Saarbrucken, Germany. bozek@mpi-inf.mpg.de|Max Planck Institute for Computer Science, Saarbrucken, Germany. bozek@mpi-inf.mpg.de|Max Planck Institute for Computer Science, Saarbrucken, Germany. bozek@mpi-inf.mpg.de|Max Planck Institute for Computer Science, Saarbrucken, Germany. bozek@mpi-inf.mpg.de