Retroactive data structures

Retroactive data structures is a way to store data so that changes to the data can be efficiently reversed (or "undone"). This means that if you make a change to the data, you can go back and look at the way it was before you made the change. To do this, the data is saved in a special way that keeps a record of the changes. That way, you can go back and explore how the data has changed over time.