protocol for using InputRanges

[Jakob Ovrum]

On Sunday, 23 March 2014 at 00:50:34 UTC, Walter Bright wrote:
> 2. Can r.front be called n times in a row? I.e. is calling 
> front() destructive?
>
> If true, this means that r.front will have to cache a copy in 
> many cases.

If `front` was destructive there would be little point in having 
it separate from `popFront`. I think it must be non-destructive 
to make sense.