The computer readable storage medium 10, wherein {HEAD ~ the operation} is parameterized by an element type parameter and {TAIL ~ a key type parameter} 
The computer readable storage medium 10 , wherein { HEAD ~ the operation } is parameterized by an element type parameter and { TAIL ~ a key type parameter }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL