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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def show_mode_indicator(viewer, tf, corner='ur'):
 """Show a keyboard mode indicator in one of the corners. Parameters viewer : an ImageView subclass instance If True, show the color bar; else remove it if present. tf : bool If True, show the mark; else remove it if present. corner : str One of 'll', 'lr', 'ul' or 'ur' selecting a corner. The default is 'ur'. """

    tag = '_$mode_indicator'

    canvas = viewer.get_private_canvas()
    try:
        indic = canvas.get_object_by_tag(tag)
        if not tf:
            canvas.delete_object_by_tag(tag)
        else:
            indic.corner = corner

    except KeyError:
        if tf:
            # force a redraw if the mode changes
            bm = viewer.get_bindmap()
            bm.add_callback('mode-set',
                            lambda *args: viewer.redraw(whence=3))

            Indicator = canvas.get_draw_class('modeindicator')
            canvas.add(Indicator(corner=corner),
                       tag=tag, redraw=False)

    canvas.update_canvas(whence=3)