{HEAD ~ R3} represents {TAIL ~ methyl}, ethyl, propyl, isopropyl, butyl, isobutyl, sec.butyl, hydroxymethyl, hydroxyethyl, mercaptomethyl, or methylthioethyl or 
represents
{ HEAD ~ R3 } represents { TAIL ~ methyl } , ethyl , propyl , isopropyl , butyl , isobutyl , sec.butyl , hydroxymethyl , hydroxyethyl , mercaptomethyl , or methylthioethyl or
HEAD HEAD HEAD HEAD HEAD 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