Acknowledgments

We thank Tijmen Jan Moser, one anonymous associate editor, and two 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. 41522404 and 41430322) and National Key Research and Development Program of China (Grant No. 2016YFC0600505). All results are reproducible in the Madagascar open-source software environment (Fomel et al., 2013).