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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 _create_subset_file(in_file, het_region_bed, work_dir, data):
 """Subset the VCF to a set of pre-calculated smaller regions. """

    cnv_regions = shared.get_base_cnv_regions(data, work_dir)
    region_bed = bedutils.intersect_two(het_region_bed, cnv_regions, work_dir, data)
    out_file = os.path.join(work_dir, "%s-origsubset.bcf" % utils.splitext_plus(os.path.basename(in_file))[0])
    if not utils.file_uptodate(out_file, in_file):
        with file_transaction(data, out_file) as tx_out_file:
            regions = ("-R %s" % region_bed) if utils.file_exists(region_bed) else ""
            cmd = "bcftools view {regions} -o {tx_out_file} -O b {in_file}"
            do.run(cmd.format(**locals()), "Extract regions for BubbleTree frequency determination")
    return out_file