1. Yale-NUS College; 6 College Avenue East Singapore 138614 Singapore
2. Department of Mathematics; National University of Singapore; 10 Lower Kent Ridge Road Singapore 119076 Singapore
3. School of Computing, Engineering and Mathematics; University of Western Sydney; Locked Bag 1797 Penrith NSW 2751 Australia
4. Institut de Mathématiques de Bourgogne, UMR 5584 du CNRS; Université de Bourgogne; B.P. 47870 Dijon cedex 21078 France