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