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