If you guarded your code with `isinstance(foo, Sequence)` then I could not use it with my `Foo` even if my `Foo` quacked like a sequence. I was forced to use nominal typing; inheriting from Sequence, or explicitly registering as a Sequence.

If I'm reading the library correctly, this is correct--but, perhaps, it could be remedied by adding a __subclasshook__ to Sequence that looked for an __iter__ attribute.  That technique might also apply to other ABCs in collections.abc, Mapping for example.  Would that work, or am I missing an critical detail?

Cheers,

/arry