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