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The "writer parser" is a parser that produces a writer AST rather than a zclsyntax AST. This can be used to produce a writer AST from existing source in order to modify it before writing it out again. It's implemented with the somewhat-unintuitive approach of running the main zclsyntax parser and then mapping the source ranges it finds back onto the token sequence to pull out the raw tokens for each object. This allows us to avoid maintaining two parsers but also keeps all of this raw-token-wrangling complexity out of the main parser.