range and algorithm-related stuff

[Christopher Wright]

Denis Koroskin wrote:
> My point is, if empty() returns false (and you can't know it because 
> empty() is not const) you may get either an invalid result, access 
> violation or an exception on member access:
> 
> void foo(Range)(const Range r)
> {
>    // is r empty? can I /safely/ access its head?
>    // let's cross our fingers and try it
>    // hoping that it is either not empty or throws an exception
>    auto head = r.head;
>    ...
> }
> 

Then it's good design to implement empty() as const. In some cases, you 
might not be able to (I believe someone mentioned IO streams as such a 
case). There, empty() might also throw.