. "Examples of the substituents represented by R6 include, for example, an alkyl group (preferably an alkyl group having from 1 to 20, more preferably from 1 to 12, and particularly preferably from 1 to 8, carbon atoms, e.g., methyl, ethyl, isopropyl, tert-butyl, n-octyl, n-decyl, n-hexadecyl, cyclopropyl, cyclopentyl, cyclohexyl), an alkenyl group (preferably an alkenyl group having from 2 to 20, mo" . . .