But for your convenience you are proposing a problematic change (see posts by others).
I think we've established it's problematic to add it to the Sequence ABC, but in terms of only adding it to specific sequences like lists, I don't know what other posts you're referring to. Why is it problematic?
Maybe I meant overgeneralization instead.
Again, the goal isn't to generalize. I'm not trying to make lists look like dicts. It's not even possible to get close - `if i in x: print(x[i])` does very different things for lists and dicts.