| http://www.w3.org/ns/prov#value | - R2 denotes hydrogen, a hydrocarbon radical having 1 to 10, preferably 1 to 4, carbon atoms, such as alkyl, phenyl, benzyl or naphthyl, an ester group containing the radical R4 of a monohydric alcohol having up to 12 carbon atoms, -CN, -NO2 or a CO-NHR1 or CO-R1 group, and R3 has the same meaning as R2 and is identical with or different from the latter, and,
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