| http://www.w3.org/ns/prov#value | - In the above formula, R1 to R4 may be the same or different and are each a hydrogen atom, a hydrocarbon group, an oxygen-containing hydrocarbon group or a nitrogen-containing hydrocarbon group, at least one of R1 to R4 is a group other than a hydrogen atom, and two or more of R1 to R4 may be bonded to form a ring.
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