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tokens: The author thanks F. Vincenzo, and V. Grieco for several fruitful discussions, and F. Matteucci for many useful suggestions and for reading the manuscript. The author also thanks an anonymous referee whose comments noticeably improved the paper. The work was supported by PRIN MIUR 2010-2011, project ‘The Chemical and dynamical Evolution of the Milky Way and Local Group Galaxies’, prot. 2010LY5N2T.