Institute of Computer Science, University of Göttingen, Goldschmidtstrasse 7, Greifswald, Germany. keller@cs.uni-goettingen.de|Institute of Computer Science, University of Göttingen, Goldschmidtstrasse 7, Greifswald, Germany. keller@cs.uni-goettingen.de|Institute of Computer Science, University of Göttingen, Goldschmidtstrasse 7, Greifswald, Germany. keller@cs.uni-goettingen.de|Institute of Computer Science, University of Göttingen, Goldschmidtstrasse 7, Greifswald, Germany. keller@cs.uni-goettingen.de