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