a user interface for receiving user input to enable tracking wherein the user input is received via a user interface that enables the user to manually enable {HEAD ~ tracking}, to disable tracking or enable {TAIL ~ automatic triggering} of tracking 
to enable
a user interface for receiving user input to enable tracking wherein the user input is received via a user interface that enables the user to manually enable { HEAD ~ tracking } , to disable tracking or enable { TAIL ~ automatic triggering } of tracking
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH REL OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH