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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
http://www.cis.upenn.edu/~xtag