{HEAD ~ R3} will represent {TAIL ~ alkyl}, substituted alkyl, aryl or substituted aryl and its formula will be determined by the make up of R1 and R2 
{ HEAD ~ R3 } will represent { TAIL ~ alkyl } , substituted alkyl , aryl or substituted aryl and its formula will be determined by the make up of R1 and R2
HEAD HEAD HEAD HEAD HEAD 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