Long-distance dependencies: a problem for subcategorization and agreement.

(1) What cities does Continental service?

(2) What flights do you have from Boston to Baltimore?

(3) What time does that flight leave Atlanta?

(4) Which flight do you want me to have the travel agent book?

(5) Which flight did the travel agent say leaves at 6?

Strategy:

Introduce a new set of features with the attribute GAP (or SLASH) and a value corresponding to the "missing" grammatical category: GAP NP, GAP PP etc. This is a "head feature" which will "percolate" up the tree.

When the value of GAP unifies with a matching constituent, the gap is "filled". A "complete sentence" should contain no unfilled gaps. (*Do you have from Boston to Baltimore)

Typing:

It's useful in a unification grammar to specify the possible values for an attribute. This can be conveniently represented in a "type hierarchy" (a "lattice", to allow for multiple inheritence). This allows for constitency checking and the elimination of some redundancies. It is also useful for lexicon building.