def print_token(self, token_node_index):
        """returns the string representation of a token."""
        err_msg = "The given node is not a token node."
        assert isinstance(self.nodes[token_node_index], TokenNode), err_msg
        onset = self.nodes[token_node_index].onset
        offset = self.nodes[token_node_index].offset
        return self.text[onset:offset]
returns the string representation of a token.