| http://www.w3.org/ns/prov#value | - chosen from linear and branched alkyl groups comprising from 1 to 6 carbon atoms, such as 2 or 3 carbon atoms, and hydroxyalkyl groups comprising from 1 to 4 carbon atoms; R.sub.4, R.sub.5 and R.sub.6, which may be identical or different, are each chosen
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