>
> Now, I just tried to use my super term union, but I ran
> into a problem. It looks like a fault in type inference,
> something like this:
>
> let f (x:[`A|`B]) = ..
> let g (x:[`A]) = ..
> ..
> fun x ->
>    f x;
>    g x;
>    ..
>
You have to define it as follows:

let f (x : [< `A | `B ]) = ..
let g (x : [`A]) =

to let the compiler know that the argument of f could be refined  
further.