> The refactoring could be a small but limited program of its own, that is evaluated against the program abstract syntax graph, and that can be as powerful as i

> The refactoring could be a small but limited program of its own, that is evaluated against the program abstract syntax graph, and that can be as powerful as is warranted or needed to properly transform programs using the code. Moving or renaming code could be a relatively simple type of refactoring. However, if you've renamed f(int) to g(int, int) such that callers of f(N) should call g(N, 0), then a slightly more complex refactoring script could handle that too. Go has such a tool already in the form of `gofmt -r`. You give it a semantic rewrite rule like `a[b:len(a)] -> a[b:]` and it runs through the AST and rewrites the code for you. When go was in development (pre go 1.0) the developers used this mechanism quite often to make backwards incompatible changes to the language.