[Ur] Opaque ascription

[Adam Chlipala]

Ron de Bruijn wrote:
> The Ur/Web module system doesn't support opaque ascription currently. 
> Is there a reason for that considering that the compiler 
> implementation makes fairly extensive use of this Standard ML feature?

Can you explain more what you want (maybe with an example)?  I would use 
the word "opaque" to describe the only signature ascription operator 
that Ur includes.