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