Let's never assume we can know what anyone will ever legitimately want to do. One of the use cases of preprocessors is to accommodate several versions of the l

Let's never assume we can know what anyone will ever legitimately want to do. One of the use cases of preprocessors is to accommodate several versions of the language. Obviously, if the AST changes between versions, then no AST rewriter can help. We have had to bake our own ad-hoc simple preprocessor for OCaml to accommodate various versions of its stdlib API, for instance.