Indexed Language Article Index for
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Indexed Language




Indexed languages and are a proper Subset of Context-sensitive Language s and a proper superset of Mildly Context-sensitive Language s and Context-free Language s; they are closed under union, concatenation, and the Kleene Star , but are not closed under intersection or complement.

The class of indexed languages has practical importance in Natural Language Processing as a computationally affordable generalization of Context-free Languages , since Indexed Grammar s can describe many of the nonlocal constraints occurring in natural languages.


EXAMPLES


The following languages are indexed, but are not Mildy Context Sensitive nor Context-free :