Chapter One of "Highly Frustrated Magnetism"
Eds. C. Lacroix, P. Mendels, F. Mila, Springer Series in Solid State Sciences 164 (2011).
Published in "Topological Aspects of Low-Dimensional Systems", Eds. A. Comtet et al. (Les Houches, Session LXIX).
ICTP Trieste, July 2011
Colloquium, ENS Lyon, September 2010
Colloquium, Weizmann Institute of Science, June 2009