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