1. Chair of Discrete Mathematics/Geometry, Technical University Berlin, Berlin 10623, Germany
2. Max Planck Institute for Mathematics in the Sciences, Leipzig 04103, Germany
3. Department of Biosystems Science and Engineering, Federal Institute of Technology (ETH Zürich), Basel 4058, Switzerland
4. Swiss Institute of Bioinformatics, Basel 4058, Switzerland
5. Department of Biosphere Sciences and Engineering, Carnegie Institution for Science, Baltimore, MD 21218
6. Department of Biology, Johns Hopkins University, Baltimore, MD 21218