| http://www.w3.org/ns/prov#value | - According to an embodiment of interest, one of the radicals R.sub.2and R.sub.3 is hydroxy-, sulfo-, sulfato-, cyano- or carboxy-substituted C.sub.1 -C.sub.4 alkyl, and the other of the radicals R.sub.2 and R.sub.3 is hydrogen or C.sub.1 -C.sub.4 alkyl, especially hydrogen.R.sub.2 and R.sub.3 are especially preferably each independently of the other hydrogen or C.sub.1 -C.sub.4 alkyl, especially hy
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