Check if the nonterminal is valid.
        :param nonterm: Nonterminal to check.
        :raise NotNonterminalException: If the object doesn't inherit from Nonterminal class.

def _control_nonterminal(nonterm):
        # type: (Type[Nonterminal]) -> None
        """
        Check if the nonterminal is valid.
        :param nonterm: Nonterminal to check.
        :raise NotNonterminalException: If the object doesn't inherit from Nonterminal class.
        """
        if not inspect.isclass(nonterm) or not issubclass(nonterm, Nonterminal):
            raise NotNonterminalException(nonterm)