{HEAD ~ A p code frame} or {TAIL ~ simply frame} represents a unit of action or operation in the system 
{ HEAD ~ A p code frame } or { TAIL ~ simply frame } represents a unit of action or operation in the system
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH