Returns the root hash of this tree. (Only re-computed on change.)

def root_hash(self):
        """Returns the root hash of this tree. (Only re-computed on change.)"""
        if self.__root_hash is None:
            self.__root_hash = (
                self.__hasher._hash_fold(self.__hashes)
                if self.__hashes else self.__hasher.hash_empty())
        return self.__root_hash