This is awesome and I've needed it often. Could it hypothetically support filtered undo? For example in a collaborative app where not all edits are owned by the user.

I think you can do that with `undoredo` by doing all the following:

- Having one history bistack for each user or permissions group to enforce ownership;

- Giving each new object's a unique key (e.g. from a counter, possibly a persistent one) to prevent key collisions by avoiding ever trying to fill the same container slot more than once;

- Storing the objects in a sparse container such as `BTreeMap`, `BTreeSet`, `HashMap`, `HashSet` to avoid having to visit dead keys when iterating over all the elements.

I have not tested this, but it could be a nice case to add to examples.

(having used a bullet list I immediately feel the need to explain myself: this message was whole written by hand)