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Pumping lemma and Ogden lemma for displacement context-free grammars
The pumping lemma and Ogden lemma offer a powerful method to prove that a
particular language is not context-free. In 2008 Kanazawa proved an analogue of
pumping lemma for well-nested multiple-context free languages. However, the
statement of lemma is too weak for practical usage. We prove a stronger variant
of pumping lemma and an analogue of Ogden lemma for this language family. We
also use these statements to prove that some natural context-sensitive
languages cannot be generated by tree-adjoining grammars.Comment: Shortened version accepted to DLT 2014 conferenc