Case C-303/12: Judgment of the Court (Fifth Chamber) of 12 December 2013 (request for a preliminary ruling from the Tribunal de première instance de Liège (Belgium)) — Guido Imfeld, Nathalie Garcet v État belge (Freedom of establishment — Equal treatment — Income tax — Legislation for the avoidance of double taxation — Income earned in a State other than the State of residence — Method of exemption subject to progressivity in the State of residence — Account taken, in part, of personal and family circumstances — Loss of certain tax advantages linked to the personal and family circumstances of the worker)