[Ur] Empty con illegal?

[Adam Chlipala]

Ron de Bruijn wrote:
> The following is currently illegal according to the compiler, but I 
> believe that it shouldn't be.
>
> a.ur
> con wth = []
>
> a.urs
> con wth = []

The only error message I get is about undetermined unification 
variables, which makes sense, since [wth] could have kind [{K}] for any 
[K].  Your example is underconstrained; Ur programs are not allowed to 
have any unresolved polymorphism.