> This one is common to both python bool and np.bool_.

It's not, though. One of my examples was using `~` (`__invert__`) from the proposed ABC). And the behaviour of that function differs between NumPy and Python

I think one of the rules about this sort of abstraction should be that there should be multiple concrete examples available that you intend the abstraction to apply to, *before* you abstract. Otherwise there's a risk of making an abstraction that doesn't turn out to fit actual use-cases. Right now, we only have Python's bool as a concrete example; NumPy's bool_ is IMO not close enough in its behaviour. Are you aware of other bool-like types around that the proposed ABC would be helpful for?