Since {TAIL ~ the number} of event execution units decreases, the number of times an event execution unit is registered in an event queue and {HEAD ~ a registered event} is extracted from the event queue decreases 
Since { TAIL ~ the number } of event execution units decreases , the number of times an event execution unit is registered in an event queue and { HEAD ~ a registered event } is extracted from the event queue decreases
OTH TAIL TAIL TAIL TAIL TAIL TAIL 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 HEAD HEAD OTH OTH OTH OTH OTH OTH OTH