This research was funded by the German Federal Ministry for Research and Technology (BMBF) under Grant #01IV101A0 and by the German Research Association (DFG) under Grant DFG Ha 1026/6-3 and Grant DFG We 1468/4-1. We would like to thank S. Haack, M. Löchel, M. Meurer, U. Sauerland, and M. Schrattenholzer for their work on SCREEN; as well as David Bean, Alexandra Klein, Steven Minton, Johanna Moore, Ellen Riloff and five anonymous referees for comments on earlier versions of this paper.