Additionally, in the described embodiment, {HEAD ~ a dynamic ad placement engine} knows about the current context of {TAIL ~ the system} before an ad request is received 
Additionally , in the described embodiment , { HEAD ~ a dynamic ad placement engine } knows about the current context of { TAIL ~ the system } before an ad request is received
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH