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