1. Rotterdam School of Management, Erasmus University, 3062 PA Rotterdam, Netherlands; and Delft Institute of Applied Mathematics, Delft University of Technology, 2628 CD Delft, The Netherlands
2. Delft Institute of Applied Mathematics, Delft University of Technology, 2628 CD Delft, The Netherlands; and Centrum Wiskunde and Informatica, 1090 GB Amsterdam, The Netherlands