This is also important when {HEAD ~ the operator} is not in {TAIL ~ a position} to physically observe whether or not he has effectively closed the valve 
is not in
This is also important when { HEAD ~ the operator } is not in { TAIL ~ a position } to physically observe whether or not he has effectively closed the valve
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH