Undo

  To cancel the most recently performed function, 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 addition again
  and places the result in the accumulator.

  Additional topics:

  *   Edit Menu

  *   File Menu

  *   Mode Menu

  *   Overview of Calculator