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