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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 _run_concat_variant_files_bcftools(in_list, out_file, config, naive=False):
 """Concatenate variant files using bcftools concat, potentially using the fast naive option. """

    if not utils.file_exists(out_file):
        with file_transaction(config, out_file) as tx_out_file:
            bcftools = config_utils.get_program("bcftools", config)
            output_type = "z" if out_file.endswith(".gz") else "v"
            if naive:
                args = "--naive"
            else:
                args = "--allow-overlaps"
            cmd = "{bcftools} concat {args} -O {output_type} --file-list {in_list} -o {tx_out_file}"
            do.run(cmd.format(**locals()), "bcftools concat variants")
    if out_file.endswith(".gz"):
        bgzip_and_index(out_file, config)
    return out_file