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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 _add_or_remove_scrollbar(self, width, height, dy):
 """ Add or remove a scrollbar from this listbox based on height and available options. :param width: Width of the Listbox :param height: Height of the Listbox. :param dy: Vertical offset from top of widget. """

        if self._scroll_bar is None and len(self._options) > height:
            self._scroll_bar = _ScrollBar(
                self._frame.canvas, self._frame.palette, self._x + width - 1, self._y + dy,
                height, self._get_pos, self._set_pos)
        elif self._scroll_bar is not None and len(self._options) <= height:
            self._scroll_bar = None