| http://www.w3.org/ns/prov#value | - It has now been found that a compound of the general formula: ##STR4## wherein each of R1 and R2 is a hydrogen atom or an alkyl group having 1-4 carbon atoms, R3 and R4 are each a hydrogen atom or together form another chemical bond, each X is a hydroxyl group, a halogen atom, an alkyl group having 1-4 carbon atoms and an alkoxy group having 1-4 carbon atoms and may be the same or different, and n
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