If non-nil ask about discarding undo info for the current command.
Normally, Emacs discards the undo info for the current command if
it exceeds `undo-outer-limit'. But if you set this option
non-nil, it asks in the echo area whether to discard the info.
If you answer no, there is a slight risk that Emacs might crash, so
only do it if you really want to undo the command.
This option is mainly intended for debugging. You have to be
careful if you use it for other purposes. Garbage collection is
inhibited while the question is asked, meaning that Emacs might
leak memory. So you should make sure that you do not wait
excessively long before answering the question.