docs: def path_rules(self, from_symbol, to_symbol):
          # type: (Type[Nonterminal], Type[Nonterminal]) -> List[Type[Rule]]
          """
          Get sequence of unit rules between first and second parameter.
          :param from_symbol: From which symbol.
          :param to_symbol: To which symbol.
          :return: Sequence of unit rules. Empty sequence mean there is no way between them.
          """
          if from_symbol not in self.t or to_symbol not in self.t:
              return []
          return self.f[self.t[from_symbol]][self.t[to_symbol]] or []