code: def _slice_bam_chr21(in_bam, data):
      
      sambamba = config_utils.get_program("sambamba", data["config"])
      out_file = "%s-chr%s" % os.path.splitext(in_bam)
      if not utils.file_exists(out_file):
          bam.index(in_bam, data['config'])
          with pysam.Samfile(in_bam, "rb") as bamfile:
              bam_contigs = [c["SN"] for c in bamfile.header["SQ"]]
          chromosome = "21"
          if "chr21" in bam_contigs:
              chromosome = "chr21"
          with file_transaction(data, out_file) as tx_out_file:
              cmd = ("{sambamba} slice -o {tx_out_file} {in_bam} {chromosome}").format(**locals())
              out = subprocess.check_output(cmd, shell=True)
      return out_file
docs: return only one BAM file with only chromosome 21