We thank Jeffrey Shragge, Kris Innanen, 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. 41274119, 41430322),
863 Program of China (Grant No. 2012AA09A2010), and 973 Program
of China (Grant No. 2013CB429805). All results are reproducible in the
Madagascar open-source software environment (Fomel et al., 2013).