ZWEI Undo command (C-Shift-U)

[Richard M. Stallman <rms@MIT-OZ>]

I've found that few people know about the new ZWEI unlimited Undo feature.
This feature is simply that you can undo all the changes you
have made to a buffer since you read in or saved a file,
one batch of contiguous changes at a time, by typing C-Shift-U
over and over again.

Absolutely all changes are recorded.  For example, you can
undo a Query Replace, or part of one.  Each C-Shift-U gets
one replacement.

If you undo too far, type C-Shift-R to redo the changes undone.
One C-Shift-R cancels one C-Shift-U.

If you set ZWEI:*DISCARD-UNDO-INFO-ON-SAVING* to NIL,
you can undo all the way to when you visited the file,
regardless of whether you have saved it.

I found that it required an effort at first to remember
to consider using the command, but once I had developed the
habit it was a great saver.