Synchronous Rewriting in Treebanks

Several formalisms have been proposed for modeling trees with discontinuous phrases. Some of these formalisms allow for synchronous rewriting. However, it is unclear whether synchronous rewriting is a necessary feature. This is an important question, since synchronous rewriting greatly increases parsing complexity. We present a characterization of recursive synchronous rewriting in constituent treebanks with discontinuous annotation. An empirical investigation reveals that synchronous rewriting is actually a necessary feature. Furthermore, we transfer this property to grammars extracted from treebanks.