code: def _parse_argument(self):
          
          reset = self._head
          try:
              argument = self._parse(contexts.ARGUMENT_NAME)
          except BadRoute:
              self._head = reset
              raise
          self._emit_first(tokens.ArgumentOpen())
          self._emit_all(argument)
          self._emit(tokens.ArgumentClose())
docs: Parse an argument at the head of the wikicode string.