protocol for using InputRanges

[Andrei Alexandrescu]

On 3/27/14, 3:49 AM, "Marc Schütz" <schuetzm at gmx.net>" wrote:
> I was originally going to do that, but then I took a closer look at the
> documentation, which says ([1] in the documentation of `isInputRange()`):
>
> "Calling r.front is allowed only if calling r.empty has, or would have,
> returned false."

Probably we need to amend that. For efficient ranges, front() and 
popFront() should only be guaranteed to work if either empty() or 
length() were evaluated first, and they returned false or nonzero, 
respectively.

Andrei