If {HEAD ~ the group E} is {TAIL ~ a null group}, the flow advances to Step 1 5 
If { HEAD ~ the group E } is { TAIL ~ a null group } , the flow advances to Step 1 5
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH