comparing the original value of {HEAD ~ the data unit} with {TAIL ~ a value} of the update data; and 
with
comparing the original value of { HEAD ~ the data unit } with { TAIL ~ a value } of the update data ; and
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH