More broadly, it is only necessary to reject an argument type at the __array_function__ level if it defines __array_function__ itself, because that’s the only case where it would make a difference to return NotImplemented rather than trying (and failing) to call the overriden function implementation.

Yes, this makes sense -- these are the only types that could possibly change the outcome if the class now called fails to produce a result. Indeed, that reasoning makes it logical that `ndarray` itself is not present even though it defines `__array_ufunc__` - we know it cannot handle anything with a `__array_ufunc__` implementation.

Hameer, is Stephan's argument convincing to you too? If so, I'll close the PR.

-- Marten