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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 _set_meta(self):
 """ set the meta fields of the ParameterSet as those that are shared by ALL parameters in the ParameterSet. For any fields that are not """

        # we want to set meta-fields that are shared by ALL params in the PS
        for field in _meta_fields_twig:
            keys_for_this_field = set([getattr(p, field)
                                       for p in self.to_list()
                                       if getattr(p, field) is not None])
            if len(keys_for_this_field)==1:
                setattr(self, '_'+field, list(keys_for_this_field)[0])
            else:
                setattr(self, '_'+field, None)