{HEAD ~ R2} represents methyl, ethyl, {TAIL ~ propyl}, isopropyl, butyl, isobutyl, or sec.butyl 
represents
{ HEAD ~ R2 } represents methyl , ethyl , { TAIL ~ propyl } , isopropyl , butyl , isobutyl , or sec.butyl
HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH