def write(self, fptr):
        """Write a Data Reference box to file.
        """
        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)
Write a Data Reference box to file.