{HEAD ~ W} is {TAIL ~ oxygen}, sulfur or an unsubstituted or lower alkyl substituted nitrogen 
is
{ HEAD ~ W } is { TAIL ~ oxygen } , sulfur or an unsubstituted or lower alkyl substituted nitrogen
HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH