code: def _combine_coverages(items, work_dir, input_backs=None):
      
      out_file = os.path.join(work_dir, "sample_coverages.txt")
      if not utils.file_exists(out_file):
          with file_transaction(items[0], out_file) as tx_out_file:
              with open(tx_out_file, 'w') as out_f:
                  for data in items:
                      cov_file = tz.get_in(["depth", "bins", "seq2c"], data)
                      with open(cov_file) as cov_f:
                          out_f.write(cov_f.read())
                  if input_backs:
                      for input_back in input_backs:
                          with open(input_back) as in_handle:
                              for line in in_handle:
                                  if len(line.split()) >= 4:
                                      out_f.write(line)
      return out_file
docs: Combine coverage cnns calculated for individual inputs into single file.
  
      Optionally moves over pre-calculated coverage samples from a background file.