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