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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def save_data_files(bs, prefix=None, directory=None):
 """Write the phonon band structure data files to disk. Args: bs (:obj:`~pymatgen.phonon.bandstructure.PhononBandStructureSymmLine`):
 The phonon band structure. prefix (:obj:`str`, optional):
 Prefix for data file. directory (:obj:`str`, optional):
 Directory in which to save the data. Returns: str: The filename of the written data file. """

    filename = 'phonon_band.dat'
    filename = '{}_phonon_band.dat'.format(prefix) if prefix else filename
    directory = directory if directory else '.'
    filename = os.path.join(directory, filename)

    with open(filename, 'w') as f:
        header = '#k-distance frequency[THz]\n'
        f.write(header)

        for band in bs.bands:
            for d, e in zip(bs.distance, band):
                f.write('{:.8f} {:.8f}\n'.format(d, e))
            f.write('\n')

    return filename