This is merely a psychological issue, because when you're programming in an "everything-mutable" Lisp dialect, you also don't worry about this. If you don't know where else you're passing that data structure and don't care to find out, then ... you don't mutate it. Libraries and API's simply don't mutate the inputs that you pass to them, unless loudly documented otherwise. Thus, mutation is applied in controlled ways whose scope is easy to ascertain and limit by inspection.