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In linguistics, a rewrite rule for natural language (phrase structure rule, analog of production rules in formal grammars) in generative grammar is a rule of the form A → X where A is a syntactic category label, such as noun phrase or sentence, and X is a sequence of such labels and/or morphemes, expressing the fact that A can be replaced by X in generating the constituent structure of a sentence.
Rewrite rules underlie rewriting systems; there are many different varieties of rewriting systems that are commonly studied.
See also 
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