<oPopUp>:open() --> self

        open() is a method of the PopUpMenu class that is used for activating
        a pop-up menu.  When called, open() performs two operations.  First,
        open() saves the previous contents of the region of the screen that
        the pop-up menu occupies.  Second, open() calls its pop-up menu's
        display() method.

        Note:  This message only has meaning when the pop-up menu is
        closed.