Commands that modify the text in a buffer can be undone, so that the text reverts to its state before the command was invoked, using Undo. Details of modifying commands are kept in an undo ring so that previous commands can be undone. The undo ring works like a stack, in that commands are pushed onto the ring and can only be popped off on a "last in first out" basis.
Un-Kill can also be used to replace text that has inadvertently been deleted.
Arguments: None
Key sequence: Ctrl+Shift+_
Undoes the last command. If invoked repeatedly, the most recent commands in the editing session are successively undone.
See also: Clear Undo, Toggle Global Simple Undo
The number of items in the undo ring.
LispWorks Editor User Guide (Macintosh version) - 9 Dec 2014