1. Operations Research Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142
2. Yale School of Management, Yale University, New Haven, Connecticut 06511
3. Operations Research Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142; and Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142
4. Operations Research Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142; and Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142