undoundo is an interactive compiled Lisp function in `
simple.el'. It is bound to C-_, <undo>, C-/, C-x u, <menu-bar> <edit> <undo>.
(undo &optional ARG) Undo some previous changes. Repeat this command to undo more changes. A numeric ARG serves as a repeat count. In Transient Mark mode when the mark is active, only undo changes within the current region. Similarly, when not in Transient Mark mode, just C-u as an argument limits undo to changes within the current region.