The second control signal is coupled to an input of {HEAD ~ the NAND gate} and inactivates {TAIL ~ the first control signal} 
The second control signal is coupled to an input of { HEAD ~ the NAND gate } and inactivates { TAIL ~ the first control signal }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL