splitter string/char different behavior

[SrMordred]

> In order to know where to split, it really has to do it from 
> the front. If it starts from the back, you won't necessarily 
> split in the same places as when iterating from the front, and 
> that would violate how bidirectional ranges are supposed to 
> work (the elements should be the same - just in reverse - if 
> you iterate from the back). That being the case, it makes sense 
> that splitting on a single element would result in a range that 
> was bidirectional, whereas splitting on a range of elements 
> would result in a range that's only a forward range.
>
> - Jonathan M Davis

Nice!
since dropBack is a BidirectionalRange everything make sense now.
Thanks everybody!

I just think that the error message should be a little better, 
since I have no idea about the incompatible Range types looking 
only to the error message. (Dont know if is possible, but 
anyway.. )