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