The region also includes an acid resistant material portion provided directly over {HEAD ~ the acetal material portion} in an amount such that the combination of the thickness of the acetal material portion and the acid resistant material portion is substantially equal to {TAIL ~ the certain overall thickness} 
The region also includes an acid resistant material portion provided directly over { HEAD ~ the acetal material portion } in an amount such that the combination of the thickness of the acetal material portion and the acid resistant material portion is substantially equal to { TAIL ~ the certain overall thickness }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL