[Python-3000] Abilities / Interfaces

[Andrew Koenig]

> What does it add to have to declare a class as being "Iterable", if it
> already implements __iter__?  What does the notion of "Iterable" add to
> the execution *or* understanding of the code?

Let's suppose for the sake of argument that declaring a class as being
"Iterable" adds nothing.  What does that say about whether the ability to
declare other abilities is useful?  I think it doesn't say much.