| http://www.w3.org/ns/prov#value | - wherein each of the groups R.sup.1, R.sup.2, R.sup.3 and R.sup.4, which are independent of one another, is a methyl group, an ethyl group or a n-propyl group, provided that two groups selected from the groups R.sup.1 to R.sup.4 may be connected
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