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<p>in which the left-hand side of each production rule consists of only a single nonterminal symbol. This restriction is non-trivial; not all languages can be generated by context-free grammars. Those that can are called <a href="page.php?w=context-free_language">context-free language</a>s. These are exactly the languages that can be recognized by a non-deterministic <a href="page.php?w=push_down_automaton">push down automaton</a>. Context-free languages are the theoretical basis for the syntax of most <a href="page.php?w=programming_languages">programming languages</a>.</p><p>
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