{HEAD ~ Data} coding for enforcing {TAIL ~ constraints} on ones and zeros in a communications channel 
coding for enforcing
{ HEAD ~ Data } coding for enforcing { TAIL ~ constraints } on ones and zeros in a communications channel
HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH