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