Recently, it was shown (Kuhlmann, Satta: Tree-adjoining grammars are not closed under strong lexicalization. Comput. Linguist., 2012) that finitely ambiguous tree adjoining grammars cannot be transformed into a normal form (preserving the generated tree language), in which each production contains a lexical symbol. A more powerful model, the simple context-free tree grammar, admits such a normal form. It can be effectively constructed and the maximal rank of the nonterminals only increases by 1. Thus, simple context-free tree grammars strongly lexicalize tree adjoining grammars and themselves.
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