The authors are grateful to Sergey Fomel for inspiring
discussions. We thank the associate editor, Joseph Dellinger, and
four anonymous reviewers for helpful suggestions, which improved the
quality of the paper. This work is supported by National Natural
Science Foundation of China (Grant No. 41774127 and 41522404) 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).