docs: def _fetch_output_files(self, retrieved):
          """
          Checks the output folder for standard output and standard error
          files, returns their absolute paths on success.
  
          :param retrieved: A dictionary of retrieved nodes, as obtained from the
            parser.
          """
          from aiida.common.datastructures import calc_states
          from aiida.common.exceptions import InvalidOperation
          import os
  
          # check in order not to overwrite anything
          #         state = self._calc.get_state()
          #         if state != calc_states.PARSING:
          #             raise InvalidOperation("Calculation not in {} state"
          #                                    .format(calc_states.PARSING) )
  
          # Check that the retrieved folder is there
          try:
              out_folder = retrieved[self._calc._get_linkname_retrieved()]
          except KeyError:
              raise IOError("No retrieved folder found")
  
          list_of_files = out_folder.get_folder_list()
  
          output_path = None
          error_path = None
  
          if self._calc._DEFAULT_OUTPUT_FILE in list_of_files:
              output_path = os.path.join(out_folder.get_abs_path('.'),
                                         self._calc._DEFAULT_OUTPUT_FILE)
          if self._calc._DEFAULT_ERROR_FILE in list_of_files:
              error_path = os.path.join(out_folder.get_abs_path('.'),
                                        self._calc._DEFAULT_ERROR_FILE)
  
          return output_path, error_path