{HEAD ~ R16} and R17 are {TAIL ~ each}, independently, hydrogen, alkyl of 1 12 carbon atoms, or n Ar 
{ HEAD ~ R16 } and R17 are { TAIL ~ each } , independently , hydrogen , alkyl of 1 12 carbon atoms , or n Ar
HEAD HEAD HEAD HEAD HEAD OTH OTH REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH