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I am grateful to Peter Aichelburg, Herbert Balasin, Robert Bartnik, Robert Beig, Chiang-Mei Chen, Piotr ChruĊciel, Sergio Dain, Jörg Frauendiener, Helmut Friedrich, Sean Hayward, Jacek Jezierski, Jerzy Kijowski, Stephen Lau, Lionel Mason, Niall Ó Murchadha, James Nester, Ezra Newman, Alexander Petrov, Walter Simon, George Sparling, Jacek Tafel, Paul Tod, Roh-Suan Tung, Helmuth Urbantke, Juan Antonio Valeinte-Kroon, James Vickers, Mu-Tao Wang and Jong Yoon for their valuable comments and stimulating questions. Special thanks to Jörg Frauendiener for continuous and fruitful discussions in the last sixteen years, to James Nester for the critical reading of an earlier version of the present manuscript, whose notes and remarks considerably improved its clarity, and to the two referees whose constructive criticism helped to make the present review more accurate and complete. Thanks are due to the Erwin Schrödinger Institute, Vienna, the Stefan Banach Center, Warsaw, the Universität Tübingen, the Max-Planck-Institut für Mathematik in den Naturwissenschaften, Leipzig, the National Center for Theoretical Sciences, Hsinchu, and the National Central University, Chungli, for hospitality, where parts of the present work were done and/or presented. This work was partially supported by the Hungarian Scientific Research Fund (OTKA) grants T042531 and K67790.
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