MH takes pleasure in thanking Othmar Brodbeck, Gary Gibbons, Domenico Giulini, Dieter Maison, Gernot Neugebauer, Norbert Straumann, Michael Volkov and Robert Wald for helpful discussions. PTC acknowledges bibliographical advice from Michael Volkov.
This work was supported by SNSF Grant P-21-41840.94 (MH), by a Polish Ministry of Science and Higher Education grant Nr N N201 372736 grant (PTC), by projects PTDC/MAT/108921/2008 and CERN/FP/116377/2010, and by CAMSDG, through FCT plurianual funding (JLC).
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