tokens: Acknowledgments The author gratefully acknowledges support from the ECOGAL project, grant agreement 291227, funded by the European Research Council under ERC-2011-ADG, and warmly thanks David Armstrong, Andrew Collier-Cameron, Ian Dobbs-Dixon, David Kipping, Joe Llama, Annelies Mortier and Hugh Osborn for stimulating discussions, and Tyler Robinson for an insightful review of this manuscript. This research has made use of NASA's Astrophysics Data System bibliographic services.
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