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