First Author:   Daniel Sgroi University of Cambridge daniel.sgroi@econ.cam.ac.uk   Austin Robinson Building, Sidgwick Avenue Cambridge, CB3 9DD, UK       http://www.econ.cam.ac.uk/faculty/sgroi/ Second Author: Emanuele Giovannetti University of Cambridge e.giovannetti@econ.cam.ac.uk   Austin Robinson Building, Sidgwick Avenue Cambridge, CB3 9DD, UK       http://www.econ.cam.ac.uk/cocombine/