| http://www.w3.org/ns/prov#value | - formula (I) R.sup.1 is hydrogen or C.sub.1 to C.sub.18 -alkyl, preferably hydrogen or C.sub.1 to C.sub.4 -alkyl and especially hydrogen, R.sup.2 and R.sup.3 are either the same and represent hydrogenor C.sub.1 to C.sub.5 -alkyl, preferably hydrogen or methyl and especially hydrogen, in which case R.sup.4 is methyl.
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