| http://www.w3.org/ns/prov#value | - wherein M1 and M2 are independently a valence bond, --O-- or --NH--, R1 is a valence bond, a group of the formula ##STR23## with the proviso that--if R1 is a valence bond--only one of M1 and M2 can be --O--, R2 and R3 are independently ##STR24## the aryl radicals R1 , R2 and R3 are optionally further substituted by alkyl of 1 to 4 carbon atoms, R4 is a valence bond, --O--, alkylene of
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