tokens: MGBA acknowledges the financial support from FAPESP 2011/23996-9 and from the FAPESP Thematic Project 2013/26258-4. MGBA is also grateful to the Kapteyn Astronomical Institute for the hospitality and to R. Misra, M. Linares, P. Bult and T. Strohmayer for the useful comments. DA acknowledges support from the Royal Society. The authors are grateful to the unknown referee for his/her meticulous review of this paper whose comments significantly increased the value of our work.
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