Let's say you have definitions like that:

    f: Nat -> Nat
    g: Nat -> Nat
    h: Nat -> Nat
    h x = g (x * 2)
    g x = f (x * 3)
    f x = x < 0 ? 1 : h (x / 4)

And now you change f to be

    f x = x < 1 ? 1 : h (x / 4)

How do you do that? There's a cycle in the callgraph. In fact - how do you calculate a hash of a function that calls itself if you need its hash to calculate its hash :)

EDIT: nevermind, recursion is a special case handled differently.