We thank Valentina Socco, Adriana Citlali Ramirez, and three anonymous
reviewers for helpful suggestions, which improved the quality of the
paper. This work is partly supported by National Natural Science
Foundation of China (Grant No. 41430322 and 41274119) and 863
Programme of China (Grant No. 2012AA09A2010). All results are
reproducible in the Madagascar open-source software environment
(Fomel et al., 2013).