Thus, the number of times {HEAD ~ gas} escapes from {TAIL ~ the valve} can be counted 
escapes from
Thus , the number of times { HEAD ~ gas } escapes from { TAIL ~ the valve } can be counted
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH