{HEAD ~ R14} and R15 are {TAIL ~ each}, independently, hydrogen, alkyl of 1 12 carbon atoms, n Ar, p NR16 R17, or p N+R16 R17 R18 Y  
{ HEAD ~ R14 } and R15 are { TAIL ~ each } , independently , hydrogen , alkyl of 1 12 carbon atoms , n Ar , p NR16 R17 , or p N+R16 R17 R18 Y
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 OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH