Moving {HEAD ~ the vent location} to {TAIL ~ either side} of the vent follower causes fluid or gas to escape from one or the other cell 
to
Moving { HEAD ~ the vent location } to { TAIL ~ either side } of the vent follower causes fluid or gas to escape from one or the other cell
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH