code: def write_nochr_reads(in_file, out_file, config):
      
      if not file_exists(out_file):
          with file_transaction(config, out_file) as tx_out_file:
              samtools = config_utils.get_program("samtools", config)
              cmd = "{samtools} view -b -f 4 {in_file} > {tx_out_file}"
              do.run(cmd.format(**locals()), "Select unmapped reads")
      return out_file
docs: Write a BAM file of reads that are not mapped on a reference chromosome.
  
      This is useful for maintaining non-mapped reads in parallel processes
      that split processing by chromosome.