"if not" condition check (for data validation)

[Denis]

On Thursday, 18 June 2020 at 17:57:49 UTC, Ali Çehreli wrote:
> Here is an earlier experiment of nested templates, which may be 
> useful in this case.
     :
> I think you should be able to pass callables as 'alias' 
> template arguments

Sounds good. This gives me an opportunity to learn how nested 
templates can be used. `alias` in this context is also new, so 
I'll dig into that too.

Thanks!