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tokens: We are grateful for stimulating and helpful discussions with Piero Madau, Monica Colpi, Mitch Begelman, Francesco Haardt, Ralph Pudritz, Chris McKee, Mike Fall, Elena M. Rossi, Melvyn Davies, Milos Milosavljevic, Kevin Schawinski, Luciano Rezzolla, Pedro Montero, Richard Larson, and Christian Reisswig. L.M., D.F., and S.B. acknowledge the hospitality of KITP, where crucial phases of this work were conceived and carried out in two different KITP programs in 2013 and 2014 (“A universe of Black Holes” and “Feedback: Gravity’s Loyal Opposition”).