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