Department of Computer Science, Free University of Berlin, Takustr. 9, Berlin, Germany. emde@inf.fu-berlin.de|Department of Computer Science, Free University of Berlin, Takustr. 9, Berlin, Germany. emde@inf.fu-berlin.de|Department of Computer Science, Free University of Berlin, Takustr. 9, Berlin, Germany. emde@inf.fu-berlin.de|Department of Computer Science, Free University of Berlin, Takustr. 9, Berlin, Germany. emde@inf.fu-berlin.de|Department of Computer Science, Free University of Berlin, Takustr. 9, Berlin, Germany. emde@inf.fu-berlin.de