1. Institut für Informatik, Universität Heidelberg, Im Neuenheimer Feld 294, Heidelberg 69120, Germany. r@hoelzl.fr.
2. Department of Mathematics, Faculty of Science, National University of Singapore, Block S17, 10 Lower Kent Ridge Road, Singapore 119076, Singapore. fstephan@comp.nus.edu.sg.
3. Department of Mathematics, Nanjing University, 22 Hankou Road, Nanjing 210093, P.R. China. yuliang.nju@gmail.com.