# 上下文无关语言和文法 ``````E  =>  E + E  =>  (E) + E  =>  (E + E) + E  =>  (a + E) + E  =>  (a + b) + E  =>  (a + b) + c
``````

``````E  =>  E + E  =>  E + c  =>  (E) + c  =>  (E + E) + c  =>  (E + b) + c  =>  (a + b) + c
``````
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