* Jonathan T. Bryant:

> # type 'a t = Cond of bool t * 'a t * 'a t | Value of 'a;;
> type 'a t = Cond of bool t * 'a t * 'a t | Value of 'a
>
> # let rec f t = match t with
>       Cond (c,t,e) -> if f c then f t else f e
>     | Value x -> x
>   ;;
> val f : bool t -> bool = <fun>

# let rec g t = match t with
      Cond (c,t,e) -> if f c then g t else g e
    | Value x -> x
  ;;
    val g : 'a t -> 'a = <fun>

The return value of f can't be generalized 'a because it's constrained
to bool in the if expression.  This problem does not arise for g.