We are grateful to Mauricio Leston who took an active part in the early stages of this work. We also acknowledge the precious insight into the subject gained through discussions and collaborations with many persons, whose (non-exhaustive) list includes Pierre Bieliavsky, Martin Cederwall, Thibault Damour, Sophie de Buyl, Francois Englert, Jarah Evslin, Luca Forte, Laurent Houart, Bernard Julia, Axel Kleinschmidt, Arjan Keurentjes, Stanislav Kuperstein, Hermann Nicolai, Bengt E.W. Nilsson, Jakob Palmkvist, Louis Paulot, Christoffer Petersson, Christiane Schomblond, Daniel H. Wesley and Peter West. In addition, we thank Jakob Palmkvist for carefully reading the manuscript and giving many useful comments. This work was supported in part by IISN-Belgium (convention 4.4511.06 (M.H. and P.S) and convention 4.4505.86 (M.H. and D.P)), by the Belgian National Lottery, by the European Commission FP6 RTN programme MRTN-CT-2004-005104 for which M.H. and D.P. are associated with VUB, and by the Belgian Federal Science Policy Office through the Interuniversity Attraction Pole P5/27.
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