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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 cutoff_w_expression(vcf_file, expression, data, name="+", filterext="", extra_cmd="", limit_regions="variant_regions"):
 """Perform cutoff-based soft filtering using bcftools expressions like %QUAL < 20 || DP < 4. """

    base, ext = utils.splitext_plus(vcf_file)
    out_file = "{base}-filter{filterext}{ext}".format(**locals())
    if not utils.file_exists(out_file):
        with file_transaction(data, out_file) as tx_out_file:
            if vcfutils.vcf_has_variants(vcf_file):
                bcftools = config_utils.get_program("bcftools", data["config"])
                bgzip_cmd = "| bgzip -c" if out_file.endswith(".gz") else ""
                intervals = ""
                if limit_regions == "variant_regions":
                    variant_regions = dd.get_variant_regions(data)
                    if variant_regions:
                        intervals = "-T %s" % vcfutils.bgzip_and_index(variant_regions, data["config"])
                cmd = ("{bcftools} filter -O v {intervals} --soft-filter '{name}' "
                       "-e '{expression}' -m '+' {vcf_file} {extra_cmd} {bgzip_cmd} > {tx_out_file}")
                do.run(cmd.format(**locals()),
                       "Cutoff-based soft filtering %s with %s" % (vcf_file, expression), data)
            else:
                shutil.copy(vcf_file, out_file)
    if out_file.endswith(".vcf.gz"):
        out_file = vcfutils.bgzip_and_index(out_file, data["config"])
    return out_file