code: def has_alias(self, header):
          
          try:
              # Force to lower case (issue #1126)
              return self._limits[self.plugin_name + '_' + header.lower() + '_' + 'alias'][0]
          except (KeyError, IndexError):
              # logger.debug("No alias found for {}".format(header))
              return None
docs: Return the alias name for the relative header or None if nonexist.