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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 list_files(tag=None, sat_id=None, data_path=None, format_str=None):
 """Produce a list of ICON EUV files. Notes ----- Currently fixed to level-2 """


    desc = None
    level = tag
    if level == 'level_1':
        code = 'L1'
        desc = None
    elif level == 'level_2':
        code = 'L2'
        desc = None
    else:
        raise ValueError('Unsupported level supplied: ' + level)

    if format_str is None:
        format_str = 'ICON_'+code+'_EUV_Daytime'
        if desc is not None:
            format_str += '_' + desc +'_'
        format_str += '_{year:4d}-{month:02d}-{day:02d}_v{version:02d}r{revision:03d}.NC'

    return pysat.Files.from_os(data_path=data_path,
                                format_str=format_str)