def METHOD_NAME(filename):
    if filename.lower().endswith(".gz"):
        import gzip
        file = gzip.open(filename, mode="rt", encoding='utf-8')
    else:
        file = open(filename, encoding='utf-8')
    chrom_line = next(line for line in file if line.startswith("#C"))
    file.close()
    headers = chrom_line.strip().split("\t")
    sequences = headers[headers.index("FORMAT") + 1:]
    # because we need 'seqs to remove' for VCF
    return sequences, sequences.copy()