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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 get_dexseq_gff(config, default=None):
 """ some older versions of the genomes have the DEXseq gff file as gff instead of gff3, so this handles that by looking for either one """

    dexseq_gff = tz.get_in(tz.get_in(['dexseq_gff', 'keys'], LOOKUPS, {}),
                           config, None)
    if not dexseq_gff:
        return None
    gtf_file = get_gtf_file(config)
    if gtf_file:
        base_dir = os.path.dirname(gtf_file)
    else:
        base_dir = os.path.dirname(dexseq_gff)
    base, _ = os.path.splitext(dexseq_gff)
    gff_file = os.path.join(base_dir, base + ".gff")
    if file_exists(gff_file):
        return gff_file
    gtf_file = os.path.join(base_dir, base + ".gff3")
    if file_exists(gtf_file):
        return gtf_file
    else:
        return None