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Augmented Backus–Naur Form (ABNF) is a metalanguage based on Backus–Naur Form (BNF)
In computer science, Augmented Backus–Naur Form (ABNF) is a metalanguage based on Backus–Naur Form (BNF), but consisting of its own syntax and derivation rules. The motive principle for ABNF is to describe a formal system of a language to be used as a bidirectional communications protocol.
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