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tokens: We acknowledge the anonymous referee for useful comments, and Tim De Zeeuw and Wyn Evans for additional comments. This material is based upon work of LC supported in part by the National Science Foundation under grant no. 1066293 and the hospitality of the Aspen Center for Physics. We acknowledge financial support from PRIN MIUR 2010-2011, project ‘The Chemical and Dynamical Evolution of the Milky Way and Local Group Galaxies’, prot. 2010LY5N2T.