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Context-free languages are commonly used to describe the structure of programming ~ languages. However many interesting problems involve not just a language's structure but also the actual usage of the language. Adding a notion of probability to ordinary grammars gives rise to probabilistic context-free grammars. Interesting in their own right because of some pretty theorems, probabilistic context-free languages can be apldied to the analysis of progrAmmiug languages, automatic parsers, and error correctors. A complete outline of the theory is presented with examples. Some open questions are posed.
C. S. Wetherell (Mon,) studied this question.