{HEAD ~ Possession} of a key for a higher service level typically includes keys for {TAIL ~ lower levels}, or low service level keys can be derived from a higher service level key but not vice versa 
{ HEAD ~ Possession } of a key for a higher service level typically includes keys for { TAIL ~ lower levels } , or low service level keys can be derived from a higher service level key but not vice versa
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH