Otherwise, {TAIL ~ an event execution unit T} having the lowest level is selected from {HEAD ~ the group E} 
Otherwise , { TAIL ~ an event execution unit T } having the lowest level is selected from { HEAD ~ the group E }
OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD