When does take modify the underlying iterator?

[cy via Digitalmars-d-learn]

On Tuesday, 16 August 2016 at 21:13:38 UTC, Steven Schveighoffer 
wrote:
> static if(isForwardRange!(typeof(iter)))

> But this may not work for any input range, since any time you 
> copy the range, you are copying internal state that may cache 
> an element or more.

Yes, that was the problem with SecretlyForwardRange, is that it 
acts like a ForwardRange because it's implicitly copied, but 
doesn't implement .save, so isForwardRange would return false.

> What we need is a take function that accepts input ranges by 
> reference, so it modifies the original.

What we need is a way to split a range into two sub-ranges, the 
head and the tail.

auto res = a.take(1)
auto head = res.head
a = res.tail

...or something. Like what findSplit does, but with an index 
instead of a delimiter.