Preferably, the opening has {HEAD ~ a first diameter} at {TAIL ~ the top surface} and a second diameter at the bottom surface, the first diameter is greater than the second diameter 
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Preferably , the opening has { HEAD ~ a first diameter } at { TAIL ~ the top surface } and a second diameter at the bottom surface , the first diameter is greater than the second diameter
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH