1. Aaditya Ramdas is Assistant Professor, Statistics and Data Science, and Machine Learning, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
2. Peter Grünwald is Head, Machine Learning Group, Centrum Wiskunde & Informatica, Amsterdam, The Netherlands, and Professor of Statistics at Leiden University, The Netherlands
3. Vladimir Vovk is Professor, Computer Science, Royal Holloway, University of London, UK
4. Glenn Shafer is University Professor, Rutgers University, Piscataway, New Jersey 08854-8019, USA