code: def start(self, s):
          # type: (Optional[Type[Nonterminal]]) -> None
          
          if s is not None and s not in self.nonterminals:
              raise NonterminalDoesNotExistsException(None, s, self)
          self._start_symbol = s
docs: Set start symbol of the grammar.
          :param s: Start symbol to set.
          :raise NonterminalDoesNotExistsException: If the start symbol is not in nonterminals.