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tokens: It is a pleasure to thank A. Grenzebach, Y. Mizuno, Z. Younsi and A. Zhidenko for useful discussions and comments. We are also grateful to the referee, D. Psaltis, for comments and suggestions that have improved the presentation. This research was partially supported by the Volkswagen Stiftung (Grant 86 866) and by the ERC Synergy Grant ‘BlackHoleCam – Imaging the Event Horizon of Black Holes’ (Grant 610058). AAA and BJA are also supported in part by the project F2-FA-F113 of the UzAS and by the ICTP through the projects OEA-NET-76, OEA-PRJ-29. AAA and BJA thank the Institut für Theoretische Physik Goethe Universität' for warm hospitality during their stay in Frankfurt.