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