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