docs: def _is_unit(rule):
      # type: (Type[Rule]) -> bool
      """
      Check if parameter is unit rule.
      :param rule: Object to check.
      :return: True if is parameter unit rule, false otherwise.
      """
      return len(rule.left) == 1 and len(rule.right) == 1 and \
             isclass(rule.fromSymbol) and isclass(rule.toSymbol) and \
             issubclass(rule.fromSymbol, Nonterminal) and issubclass(rule.toSymbol, Nonterminal)