Get list of nonterminals, what are rewritable from nonterminal passed as parameter
        using only unit rules.
        :param from_symbol: For which symbols to search.
        :return: List of nonterminals.

def reachables(self, from_symbol):
        # type: (Type[Nonterminal]) -> List[Type[Nonterminal]]
        """
        Get list of nonterminals, what are rewritable from nonterminal passed as parameter
        using only unit rules.
        :param from_symbol: For which symbols to search.
        :return: List of nonterminals.
        """
        if from_symbol not in self.t:
            return []
        reachable = []
        index = self.t[from_symbol]
        for n, i in self.t.items():
            if len(self.f[index][i] or []) > 0:
                reachable.append(n)
        return reachable