{TAIL ~ X} is {HEAD ~ sulfur} when n is 1 and oxygen when n is 1 or 2; and 
{ TAIL ~ X } is { HEAD ~ sulfur } when n is 1 and oxygen when n is 1 or 2 ; and
TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH