Raymond declared:
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> Let's be clear.  These are duck-typed methods.  A type check is inappropriate.
> Anything with o.items() is allowed regardless of type.

Wikipedia explains (http://en.wikipedia.org/wiki/Duck_typing):
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> In computer programming with object-oriented programming languages, duck typing
> is an alternative to typing. In duck typing, an object's suitability for some
> purpose is determined by the presence of certain methods and properties [...]

I did use an actual 'type' check in one of my exmaples, and that was wrong.

It is possible to do a "duck-type check" with a `hasattr(other, 'items')`.

I don't use Counter myself -- I'll try and find some real-world examples.