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