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tokens: Acknowledgements We thank Andrew Baker, Laure Ciesla, Annalisa De Cia, Loretta Dunne, Haley Gomez, Leslie Hunt, Suzanne Madden, Joshua Peek, Molly Peeples, Paola Santini, Raffaella Schneider, Jason Spyromilio, Phil Wiseman and Svitlana Zhukovska for stimulating discussions and/or providing data. We thank the referee for useful suggestions. We further acknowledge the Kavli Institute for Theoretical Physics ‘Cold Universe’ workshop where some of this work was completed. This research was supported in part by the National Science Foundation under Grant No. NSF PHY11-25915. RSS thanks the Downsbrough family for their generous support, and acknowledges support from the Simons Foundation through a Simons Investigator award.