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