Next: Inverted Sentences
This section and the sections that follow describe how the English XTAG grammar
accounts for properties of the auxiliary system described above.
In our grammar, auxiliary trees are added to the main verb tree by adjunction.
Figure 20.1 shows the adjunction tree for non-inverted
Auxiliary verb tree for non-inverted sentences: Vvx
Auxiliary trees for The music should have been being played .
The restrictions outlined in column 3 of Table 20.1 are
implemented through the features <mode>, <perfect>, <progressive> and <passive>.
The syntactic lexicon entries for the auxiliaries gives values for
these features on the foot node (VP*) in Figure 20.1. Since
the top features of the foot node must eventually unify with the
bottom features of the node it adjoins onto for the sentence to be
valid, this enforces the restrictions made by the auxiliary node. In
addition to these feature values, each auxiliary also gives values to
the anchoring node (V), to be passed up the tree to the root
VP (VPr) node; there they will become the new features for the
top VP node of the sentential tree. Another auxiliary may now adjoin
on top of it, and so forth. These feature values thereby ensure the
proper auxiliary sequencing. Figure 20.2 shows the auxiliary trees anchored by the four
auxiliary verbs in sentence ((333)). Figure 20.3 shows
the final tree created for this sentence.
The music should have been being played .
The general English restriction that matrix clauses must have tense
(or be imperatives) is enforced by requiring the top S-node of a
sentence to have <mode>=ind/imp (indicative or imperative).
Since only the indicative and imperative sentences have tense,
non-tensed clauses are restricted to occurring in embedded
Noun-verb contractions are labeled NVC in their part-of-speech field
in the morphological database and then undergo special processing to
split them apart into the noun and the reduced verb before
parsing. The noun then selects its trees in the normal fashion. The
contraction, say 'll or 'd, likewise selects the normal
auxiliary verb tree, Vvx. However, since the contracted form,
rather than the verb stem, is given in the morphology, the contracted
form must also be listed as a separate syntactic entry. These entries
have all the same features of the full form of the auxiliary verbs,
with tense constraints coming from the morphological entry (e.g. it's is listed as IT 'S NVC 3SG PRES). The ambiguous
contractions 'd (had/would) and `s (has/is)
behave like other ambiguous lexical items; there are simply multiple
entries for those lexical items in the lexicon, each with different
features. In the resulting parse, the contracted form is shown with
features appropriate to the full auxiliary it represents.
Next: Inverted Sentences