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