Allowing arbitrary types for a function's argument and return type

[pineapple via Digitalmars-d-learn]

On Thursday, 22 October 2015 at 14:36:52 UTC, John Colvin wrote:
> Using ranges instead of threads or fibers, slightly 
> over-engineered to show off features:

What does if(isIntegral!T) do? It looks like it would verify that 
the template type is a discrete number? If I were to create my 
own class, say a BigNum as an example, how could I specify that 
the isIntegral condition should be met for it?

Apart from the aesthetics, what are the functional differences 
between using recurrence and using a Generator? Will one be more 
efficient than the other?

It's not fair how easy it is to incorporate unit tests in D. Now 
what excuse will I have when my code is buggy?