Prebis pre-processes Bison grammar files to add additional functionality:
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named references
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printing of semantic values
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freeing of semantic values
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facilitating accurate garbage collection in C in the generated parser
It is no longer developed and is presented here as a historical reference. All of prebis features are now supported "natively" by recent versions of Bison. (Modern Bison doesn’t support garbage collection in C, but it does provide the mechanism to implement it in a manner similar to Prebis).
At the time when Prebis was developed (circa 2002), these features weren’t available in Bison.