Indeed, GO TO is not that unstructured; it can be said to have a fairly precise semantics of "sets the continuation of the program to <label>; does not proceed to the next statement.". Though if you want to analyze the semantics of a GO TO-using program, it's practically a required step to endow all of your labels with appropriate COME FROM annotations, marking the branches it could be reached from - and that's a whole-program analysis, at least in principle. That's the point where you're giving up modularity!