Q: How to determine that editor buffer's undo is empty

Hello lispworkers,

I wonder what the programmatic way to verify that the editor "Undo" command
will do nothing is.

BTW, I failed to find the implementation of the "Undo" command in editor
source files. Any pointers are welcome.
--
Sincerely,
Dmitri Ivanov
lisp.ystok.ru