For each digital input signal, {HEAD ~ a pair} of references is selected according to the state that represents {TAIL ~ the input signal} 
For each digital input signal , { HEAD ~ a pair } of references is selected according to the state that represents { TAIL ~ the input signal }
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL