[o11c]

Some points that often get left out of 'undo' discussions:

* The usual linked-list (or tree) implementation is very cache-unfriendly if you're undoing several steps at a time. Using "linked list inside a buffer" is better; this does not preclude trees, the back "pointer" just has to specify the offset as well as the previous buffer (when you reach the fixed-size allocation you will also have to do this). If you undo across a buffer change you'll also need to update the "most recent redo" backlink from the new buffer.

* SSO strings will usually beat external strings for small edits (this can be variable-size in the linked-list-in-a-buffer case); for large edits see if you can just incref part of the main editing buffer rope.

* It is highly useful to expose a few "shortcut" undo commands: undo to previous save, undo to previous build, etc. Manual tags is probably not very practical most of the time, and "save" is essentially one anyway.