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