The author wishes to thank Gerhard Widmer for comments on an earlier draft.
This research is supported by the projects "A New Modular Architecture for Data Mining (P12645-INF)", financed by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung (FWF), and "Universities Researching for Society: Peace and the Avoidance of Violence", financed by the Austrian Federal Ministry for Education, Science, and Culture. The Austrian Research Institute for Artificial Intelligence acknowledges basic financial support from the Austrian Federal Ministry for Education, Science, and Culture.