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