Testing for template argument being result of takeExactly

[bearophile]

Jonathan M Davis:

> So, clearly I don't have the is expression right, and this is 
> seriously pushing the edge of my knowledge of is expressions.
> So, any help would be appreciated. Thanks.

I have done some tries, but I have failed, I am sorry :-)
The is() syntax is a part of D good to burn on a campfire.
But takeExactly returns a Result struct defined inside it, so it 
even possible for the is() syntax to work on this inner (hidden?) 
type? Generally the idea of defining structs inside looks nice, 
but seems a source for troubles.

Bye,
bearophile