forming a second difference which is {HEAD ~ a function} of {TAIL ~ the second model temperature} and the first model temperature, and 
of
forming a second difference which is { HEAD ~ a function } of { TAIL ~ the second model temperature } and the first model temperature , and
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH