In one embodiment, the nodes store {HEAD ~ data} to track {TAIL ~ protection levels} of their children nodes 
to track
In one embodiment , the nodes store { HEAD ~ data } to track { TAIL ~ protection levels } of their children nodes
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH