tokens: We thank Rodolfo Barniol Duran, Ehud Nakar, Re’em Sari, Elad Steinberg, and Almog Yalinewich for helpful discussions. J.K. and R.C. would like to thank the Hebrew University and its Institute for Advanced Studies for hospitality while the key ideas of this paper were developed. We are also grateful to Ehud Nakar for a loan of office space. This work was supported by the following grants: I-CORE 1829/12, ISA 3-10417 (T.P.); NSF AST-1028111 and AST-1516299, and NASA/ATP NNX14AB43G (J.K.).
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