Context Free Grammer
In linguistics and computer science, a context-free grammar (CFG) is a 
formal grammar in which every production rule is of the form

V . w 
where V is a non-terminal symbol and w is a string consisting of terminals 
and/or non-terminals. The term "context-free" comes from the fact that the 
non-terminal V can always be replaced by w, regardless of in what context 
it occurs. A formal language is context-free if there is a context-free 
grammar that generates it.

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