"epresented by --COOR13 where R13 represents an alkyl group or an alkenyl group, or a group represented by --OCOR14 where R14 represents an alkyl group or an alkenyl group, provided that at least one of R6 to R8 is an alkyl group, a cycloalkyl group, a group represented by --COOR13, a group represented by --OCOR14, or an alkenyl group, and that when two of R6 to R8 are a hydrogen atom, then the rem" . . . .