Undoing the Most Recently Performed Function

  To cancel the most recently performed keystroke or
  operation, choose Undo from the Edit menu.  If you choose
  Undo again before entering new data, Calculator performs
  the function that you previously canceled.

  For example, if you just entered the equal sign (=), choosing
  Undo removes the result from the accumulator and places
  the last operand you entered into the accumulator.  If you
  choose Undo again, Calculator performs the operation again
  and places the result in the accumulator.

  Additional topics:

  *   Entering Data from the Keyboard

  *   Performing Arithmetic Functions

  *   Clearing the Displays

  *   Exiting Calculator

  *   Using Calculator's Menus

  *   Overview of Calculator