| http://www.w3.org/ns/prov#value | - R6 and R7, which may be the same or different, each represents a hydrogen atom, an alkyl group, a substituted alkyl group, a phenyl group, or a substituted phenyl group, wherein the substituent preferably includes a thioalkoxy group, an alkoxycarbonyl group, a hydroxy group, ##STR2## R8 represents an alkyl group, a substituted alkyl group, preferably substituted by a group such as a halogen atom,
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