Major Section: HISTORY
Example: :ubt! :x-4
The keyword command :ubt! is the same as :ubt, but with related
queries suppressed appropriately, and with a guarantee that it is
``error-free.'' More precisely, the error triple returned by :ubt!
will always have a first component of nil. :Oops will undo the last
:ubt!. See ubt, see ubu, and see u.