The most important class of them requires that non-terminals be rewritten without regard to the surrounding symbols -- which makes parsing feasible.  
The most important class of them requires that non-terminals be rewritten without regard to the surrounding symbols -- which makes parsing feasible.  An unrestricted one can be constructed for any recursively-enumerable language, but the result may not be useful in practice.  An ambiguous one would allow multiple parse-trees for some string, and regular ones describe the same set of languages as regular expressions.  FTP, name this method of specifying the syntax of languages, the transformational variety of which forms the basis of a Noam Chomsky theory.
The most important class of them requires that non-terminals be rewritten without regard to the surrounding symbols -- which makes parsing feasible.  
grammar