This relation conj:appos is used for appositions. In an apposition, conjuncts have the same referent, but two clearly different denotations, with two different points of view. conj:appos must be distinguished from conj:reform, where the second conjunct is just a reformulation of the first conjunct, elaborating on the first denotation.
The secind conjunct in apposition forms a predication in the background: in Mary, my best friend, the second conjunct my best friend is a predication on the first conjunct Mary, it is equivalent to Mary is my best friend.
Note
conj:apposis only used when the two conjuncts form two separate phrases. When the apposition is a modification, the relationmod:apposmust be used, like my friend Mary or le boulevard Saint-Michel.
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Related discussion: #59