code: def write(self, fptr):
          
          self._write_validate()
  
          # Very similar to the way a superbox is written.
          orig_pos = fptr.tell()
          fptr.write(struct.pack('>I4s', 0, b'dtbl'))
  
          # Write the number of data entry url boxes.
          write_buffer = struct.pack('>H', len(self.DR))
          fptr.write(write_buffer)
  
          for box in self.DR:
              box.write(fptr)
  
          end_pos = fptr.tell()
          fptr.seek(orig_pos)
          fptr.write(struct.pack('>I', end_pos - orig_pos))
          fptr.seek(end_pos)
docs: Write a Data Reference box to file.