code: def write_to_files_rec(instances, tokenizer, max_seq_length,
                         max_predictions_per_seq, output_files):
      
      writers = []
      for output_file in output_files:
          writers.append(
              mx.recordio.MXIndexedRecordIO(
                  os.path.splitext(output_file)[0] + '.idx', output_file, 'w'))
  
      writer_index = 0
      total_written = 0
      for (inst_index, instance) in enumerate(instances):
          features = transform(instance, tokenizer, max_seq_length, max_predictions_per_seq)
          example = features
  
          writers[writer_index].write_idx(inst_index, json.dumps(example).encode('UTF-8'))
          writer_index = (writer_index + 1) % len(writers)
          total_written += 1
  
      for writer in writers:
          writer.close()
  
      logging.info('Wrote %d total instances', total_written)
docs: Create IndexedRecordIO files from `TrainingInstance`s.