1. L3S Research Center, Leibniz University Hannover, Hannover, Germany
2. Leibniz University Hannover, Hannover, Germany
3. University of Gothenburg, Göteborg, Sweden
4. University of Bremen, Bremen, Germany
5. TIB - Leibniz Information Centre for Science and Technology, and L3S Research Center, Leibniz University Hannover, Hannover, Germany