Author contributions
Lijun Li and Hui Ni designed research, Wenjing Li and Zheyu Wu performed
experiments; Lijun Li and Wenjing Li analyzed data, Jianye Gong
performed computational experiments and analyzed, Lijun Li, Wenjing Li
and Jianye Gong wrote the paper, Yanyan Xu, Zedong Jiang, Hui Ni and
Yi-Sheng Chen modified the paper. All authors read and approved the
final manuscript.