A Generalization of Ogden's Lemma
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"Excluded positions" are incorporated into a modified form of Ogden's lemma, and a language satisfying the latter, which the authors' generalized lemma can show fails to be context-free, is presented. It is also shown that in a fairly general sense there ts no function relating the number of distinguished and excluded posnions which would allow the authors' Iteration lemma to characterize the context-free languages. Categones and SubJect Descriptors' F 4.2 [Mathematical Logic and Formal Languages]. Grammars and Other Rewrtting Systems--grammar types; F.4.3 [Mathematical Logic and Formal Languages] Formal Languages--classes defined by grammars or automata General Terms: Languages, Theory Addltmnal
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