def back(self):
        """Go back in history if possible.

        Return the undone item.

        """
        if self._index <= 0:
            return None
        undone = self.current_item
        self._index -= 1
        self._check_index()
        return undone
def back ( self ) : if self . _index <= 0 : return None undone = self . current_item self . _index -= 1 self . _check_index ( ) return undone
Go back in history if possible.

        Return the undone item.