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