Common Lisp variables are not even a little bit immutable by default. setf is the opposite of immutable values, you can mutate any place in a way that's visibl

Common Lisp variables are not even a little bit immutable by default. setf is the opposite of immutable values, you can mutate any place in a way that's visible everywhere regardless of any sort of scoping. Immutable by default might be what you'd get if CL only had let to introduce/"change" variables.