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