How to implement an "undo"?

[Zarxrax]

I would like to be able to undo adding points to an object, and am wondering what best practices I should follow?

I currently keep track of my points in a list, and I can just pop the last point off of the list, call clear_all_prompts_in_frame(), and then segment the frame again without that point. However, this doesn't work after running propagation when I am trying to make corrections to the propagated masks. What should I do in those cases?