The position of {TAIL ~ the vent} can be moved to {HEAD ~ either side} of the piston 
The position of { TAIL ~ the vent } can be moved to { HEAD ~ either side } of the piston
OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH