The system also includes a subscription mechanism that sends {HEAD ~ a subscription message} to {TAIL ~ a notification mechanism} 
The system also includes a subscription mechanism that sends { HEAD ~ a subscription message } to { TAIL ~ a notification mechanism }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL