the output of {HEAD ~ the selector element} is coupled to either the first or the second input of {TAIL ~ the selector element} as determined by said control signal, and 
the output of { HEAD ~ the selector element } is coupled to either the first or the second input of { TAIL ~ the selector element } as determined by said control signal , and
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH