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 impor- tant question, since synchronous rewrit- ing greatly increases parsing complexity. We present a characterization of recursive synchronous rewriting in constituent tree- banks with discontinuous annotation. An empirical investigation reveals that syn- chronous rewriting is actually a neces- sary feature. Furthermore, we transfer this property to grammars extracted from tree- banks.

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