code: def SetFields(fields):
      
  
      @use_context
      @use_no_input
      def _SetFields(context):
          nonlocal fields
          if not context.output_type:
              context.set_output_fields(fields)
          return NOT_MODIFIED
  
      return _SetFields
docs: Transformation factory that sets the field names on first iteration, without touching the values.
  
      :param fields:
      :return: callable