prov:value
| - in R1, R2, R3 and R4 are the same or different and each R1, R2, R3 or R4 are from the group consisting of an alkyl group having 1 to 4 carbon atoms, an alkenyl group having 2 to 4 carbon atoms and an aryl group having 6 to 12 carbon atoms or one of R1, R2, R3 and R4 is a hydrogen atom and the rest of R1, R2, R3 and R4 are the same or different and each R1, R2, R3 or R4 are from the group consistin
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