The interface provides access to {HEAD ~ the data elements} and the order keys in the dataset in {TAIL ~ a type safe manner} 
The interface provides access to { HEAD ~ the data elements } and the order keys in the dataset in { TAIL ~ a type safe manner }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL