1. Computer Science Department, University of Geneva, Geneva, Switzerland. Part of this project was done while the author was at LaBRI, CNRS, University of Bordeaux, France.
2. LaBRI, CNRS, University of Bordeaux, Bordeaux, France. Part of this project was done while the author was at Department of Informatics, Technical University of Munich, Germany.
3. Algorithmics and Computational Complexity group, Technical University of Berlin, Berlin, Germany.
4. Department of Computer Science, University of Liverpool, Liverpool L69 3BX, United Kingdom.