The authors gratefully acknowledge the department editor, Jeannette Song, the anonymous associate editor, and three anonymous referees for their highly constructive comments and suggestions, which significantly improved both the content and exposition of this paper. The authors also thank Joachim Arts, Xiuli Chao, Xin Chen, and Tim Huh for their stimulating discussions. X. Gong served as the lead author and coordinated the preparation of this paper. All authors contributed equally to this work and are listed alphabetically.
