Undo/Redo

    Undo reverses the last change made to the document. Redo undoes 
the effect of the most recent Undo. Undo applies to a variety of 
actions, some of which are on menus other than the Edit menu. 
Undo appears shaded if the most recent action cannot be undone. After 
choosing Undo, the command becomes Redo.
    The keyboard shortcut for Undo is  Cmd-Z.  

  See also: Undo/Redo, Cut, Copy, and Clear.