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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_mv_grid_district(self, mv_grid_district):
 """Adds a MV grid_district to _mv_grid_districts if not already existing"""

        # TODO: use setter method here (make attribute '_mv_grid_districts' private)
        if mv_grid_district not in self.mv_grid_districts():
            self._mv_grid_districts.append(mv_grid_district)