If the flag is true, {HEAD ~ the OptionalSort operation} changes the order keys of elements in {TAIL ~ the dataset} 
If the flag is true , { HEAD ~ the OptionalSort operation } changes the order keys of elements in { TAIL ~ the dataset }
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL