Exponential Fibonacci isn't terminating

The following input program doesn't terminate with `:trans 2` in reasonable amount of time:

```
main = fib n;

fib n = case n of
      Z -> Z
    | S(n') -> case n' of
          Z -> S(Z)
        | S(n'') -> plus (fib S(n'')) (fib n'');

plus x y = case x of
      Z -> y
    | S(x) -> S(plus x y)
```
