generating {HEAD ~ a second set} of keys for {TAIL ~ the quality layers} in the second quality layer set; and 
generating { HEAD ~ a second set } of keys for { TAIL ~ the quality layers } in the second quality layer set ; and
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH