| http://www.w3.org/ns/prov#value | - ring, wherein R.sub.3 and R.sub.4 can be joined together to form a ring, and wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 can each be joined to a phenyl ring in the central structure, a and b each, independently of the others, is an integer which is 0,
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