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