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