<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def header(self):
 """Return the bytes for the header of track. The header contains the length of the track_data, so you'll have to call this function when you're done adding data (when you're not using get_midi_data). """

        chunk_size = a2b_hex('%08x' % (len(self.track_data)
                              + len(self.end_of_track())))
        return TRACK_HEADER + chunk_size