In {TAIL ~ addition}, {HEAD ~ a subtractor} produces an error signal by subtracting the output signal from the reference input signal 
In
In { TAIL ~ addition } , { HEAD ~ a subtractor } produces an error signal by subtracting the output signal from the reference input signal
REL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH