Root restriction: A restriction is imposed on the final root
node of any XTAG derivation of a tensed sentence which equates the wh
feature and the
invlink
feature of the final root node.
If the extracted NP is not a wh-word i.e. its
wh
feature has the value -, at the end of the
derivation, Sq.b:
wh
will also have the
value -. Because of the root constraint Sq.b:
wh
will be equated to Sq.b:
invlink
which will also come to have the
value -. Then, by (
), Sr.t:
inv
will acquire the value -. This will
unify with Sr.b:
inv
which has the value
- (cf.
). Consequently, no auxiliary verb adjunction will
be forced. Hence, there will never be inversion in topicalization.
If the extracted NP is a wh-word i.e. its
wh
feature has the value +, at the end of the derivation,
Sq.b:
wh
will also have the value +. Because of
the root constraint Sq.b:
wh
will be equated
to Sq.b:
invlink
which will also come to have
the value +. Then, by (
), Sr.t:
inv
will acquire the value +. This will not unify with Sr.b:
inv
which has the value + (cf.
). Consequently, the adjunction
of an inverted auxiliary verb is required for the derivation to succeed.
Inversion will still take place even if the extraction is from an embedded
clause.
This is because the adjoined tree's root node will also have its
Sr.b:inv
set to -.
Note that inversion is only forced upon us because Sq is the
final root node and the Root restriction applies. In embedded
environments, the root restriction would not apply and the feature
clash that forces adjunction would not take place.
The
invlink
feature is not present in subject
extractions. Consequently there is no inversion in subject questions.
Subject topicalizations are blocked by setting the
wh
feature of the extracted NP to + i.e. only
wh-phrases can go in this location.