1. L3S Research Center, Leibniz University of Hannover, Hannover, Germany
2. TIB Leibniz Information Center for Science and Technology, Hannover, Germany
3. Universidad Politécnica de Madrid, Boadilla del Monte, Spain
4. Fraunhofer IAIS & University of Bonn, Dresden, Germany
5. TIB Leibniz Information Centre for Science and Technology, Hannover, Germany