Báo cáo khoa học: "Highly constrained unification grammars"

Unification grammars are widely accepted as an expressive means for describing the structure of natural languages. In general, the recognition problem is undecidable for unification grammars. Even with restricted variants of the formalism, offline parsable grammars, the problem is computationally hard. We present two natural constraints on unification grammars which limit their expressivity. We first show that non-reentrant unification grammars generate exactly the class of contextfree languages. We then relax the constraint and show that one-reentrant unification grammars generate exactly the class of tree-adjoining languages. ...