1. McCombs School of Business, The University of Texas at Austin, Austin, Texas 78705;
2. Media Laboratory at Massachusetts Institute of Technology, Cambridge, Massachusetts 02143;
3. Department of Engineering Science, University of Oxford, Oxford OX14BH, United Kingdom;
4. Grupo Interdisciplinar de Sistemas Complejos, Department of Mathematics, Universidad Carlos III de Madrid, 28911 Leganes, Spain