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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 _slice_vcf_chr21(vcf_file, out_dir):
 """ Slice chr21 of qsignature SNPs to reduce computation time """

    tmp_file = os.path.join(out_dir, "chr21_qsignature.vcf")
    if not utils.file_exists(tmp_file):
        cmd = ("grep chr21 {vcf_file} > {tmp_file}").format(**locals())
        out = subprocess.check_output(cmd, shell=True)
    return tmp_file