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tokens: We thank the anonymous referee for valuable comments and suggestions that helped us to improve the paper. We thank Elvijs Matrozis for his helpful discussions. This work is supported by the Alexander von Humboldt Foundation. R.J.S. is the recipient of a Sofja Kovalevskaja Award from the Alexander von Humboldt Foundation. B.W. acknowledges support from the 973 programme of China (No. 2014CB845700), the NSFC (Nos. 11322327, 11103072, 11033008, and 11390374), the Foundation of Yunnan province (No. 2013FB083).