In {HEAD ~ that position}, the vent is located at {TAIL ~ the interface} of the two cells 
In { HEAD ~ that position } , the vent is located at { TAIL ~ the interface } of the two cells
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH