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