Exception transparency

[Brian Goetz]

> It should be possible to write a safe compose:
>
> <F, G, H, throws X, throws Y>  Function<F, H, throws X | Y>
> compose(Function<F, G, X>  fg, Function<? super G, H, Y>  gh);
>
> And store the composed function in a variable. For this to work we
> need to have disjunctive type as a first class citizen (as Peter
> pointed in a previous message).

Can you elaborate on why this requires *first-class* disjunctive types?  The 
signature you write seems perfectly allowable within a framework that allows a 
disjunction-like entity at the use site of a throws type variable.