It also eliminates the necessity for exactly positioning {HEAD ~ the closed valve disc} to effect {TAIL ~ a leakproof closure} 
to effect
It also eliminates the necessity for exactly positioning { HEAD ~ the closed valve disc } to effect { TAIL ~ a leakproof closure }
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL