Save the transducer in the text file format of OpenFST.
        The format is specified as follows:
            arc format: src dest ilabel olabel [weight]
            final state format: state [weight]
        lines may occur in any order except initial state must be first line
        Args:
            txt_fst_file_name (str): The input file
        Returns:
            None

def load(self, txt_fst_file_name):
        """
        Save the transducer in the text file format of OpenFST.
        The format is specified as follows:
            arc format: src dest ilabel olabel [weight]
            final state format: state [weight]
        lines may occur in any order except initial state must be first line
        Args:
            txt_fst_file_name (str): The input file
        Returns:
            None
        """
        with open(txt_fst_file_name, 'r') as input_filename:
            for line in input_filename:
                line = line.strip()
                split_line = line.split()
                if len(split_line) == 1:
                    self[int(split_line[0])].final = True
                else:
                    self.add_arc(int(split_line[0]), int(split_line[1]),
                                 split_line[2].decode('hex'))