code: def _write_superbox(self, fptr, box_id):
          
          # Write the contained boxes, then come back and write the length.
          orig_pos = fptr.tell()
          fptr.write(struct.pack('>I4s', 0, box_id))
          for box in self.box:
              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 superbox.
  
          Parameters
          ----------
          fptr : file or file object
              Superbox (box of boxes) to be written to this file.
          box_id : bytes
              4-byte sequence that identifies the superbox.