That's it. You can think of an undo operation U at time (t) as a new simple command that rolls back the content to a previous version (t-x). If you undo that U operation, it's a new command U' that restores content again to version (t).

The history sequence would then be:

(t) --U-> (t-x) --U'-> (t)

And if you add (A) some new content (c) when in state (t-x), it becomes command , which can itself be undone with commands U'' (to remove A), U''' (to remove U):

(t) --U-> (t-x) --A-> (t+c) --U''--> (t-x) --U'''--> (t)