The authors thank Yan Chen, David A. Miller, and Tanya Rosenblat for countless insightful discussions. The authors also thank participants at the 2016 ESA World (Jerusalem, Israel) and ESA North American (Tucson, AZ) meetings and the 2019 Tsinghua BEAT Conference for valuable feedback. The authors are grateful to three anonymous referees and the Associate Editor, whose constructive comments greatly improved the paper. Both authors are first authors.