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