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