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tokens: We are grateful to an anonymous referee for constructive comments, and Hailiang Chen and Thomas Tauris for helpful discussions. This work was supported by the Natural Science Foundation of China under grant nos. 11133001 and 11333004, the Strategic Priority Research Program of CAS under grant no. XDB09010200, and the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence “Origin and Structure of the universe.”