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