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# Appositives, parentheticals and vocatives

These trees handle constructions where additional lexical material is only licensed in conjunction with particular punctuation marks. Since the lexical material is unconstrained (virtually any noun can occur as an appositive), the punctuation marks are anchors and the other nodes are substitution sites. There are cases where the lexical material is restricted, as with parenthetical adverbs like however, and in those cases we have the adverb as the anchor and the punctuation marks as substitution sites. When these constructions can appear inside of clauses (non-peripherally), they must be separated by punctuation marks on both sides. However, when they occur peripherally they have either a preceding or following punctuation mark. We handle this by having both peripheral and non-peripheral trees for the relevant constructions. The alternative is to insert the second (following) punctuation mark in the tokenization process (i.e. insert a comma before the period when an appositive appears on the last NP of a sentence). However, this is very difficult to do accurately.

XTAG Project
1998-09-14