1. Frankfurt School of Finance & Management, Frankfurt 60322, Germany;
2. Darden School of Business, University of Virginia, Charlottesville, Virginia 22903;
3. Microsoft Research, New York, New York 10012;
4. Department of Computer Science, Rutgers University, New Brunswick, New Jersey 08854;
5. Department of Computer Science, ETH Zurich, Zurich 8092, Switzerland