1. L3S Research Center, Leibniz University of Hannover / Hannover, Germany. lyu@L3S.de
2. L3S Research Center, Leibniz University of Hannover / Hannover, Germany. koutraki@L3S.de
3. Austrian National Library / Vienna, Austria. martin.krickl@onb.ac.at
4. L3S Research Center, Leibniz University of Hannover / Hannover, Germany
5. Amazon / Seattle, WA, USA. besnikf@amazon.com