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