| http://www.w3.org/ns/prov#value | - X' is halogen, preferably Cl, or --CN. R1 and R2 are identical or different and can be alkyl, alkoxy, alkanoyl, alkanoyloxy or alkoxycarbonyl radicals of up to 18, preferably up to 8, C atoms; moreover, one of these radicals can also be --CN, --NC, NO2, CF3 or halogen.
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