docs: def Persist(self, config_option):
      """Stores <config_option> in the writeback."""
      if not self.writeback:
        raise RuntimeError("Attempting to write a configuration without a "
                           "writeback location.")
  
      writeback_raw_value = dict(self.writeback.RawData()).get(config_option)
      raw_value = None
  
      for parser in [self.parser] + self.secondary_config_parsers:
        if parser == self.writeback:
          continue
  
        config_raw_data = dict(parser.RawData())
        raw_value = config_raw_data.get(config_option)
        if raw_value is None:
          continue
        break
  
      if writeback_raw_value == raw_value:
        return
  
      if raw_value is None:
        return
  
      self.SetRaw(config_option, raw_value)
      self.Write()