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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 get_bounding_box(self):
 """ Returns the bounding box for this cell. Returns ------- out : Numpy array[2,2] or ``None`` Bounding box of this cell [[x_min, y_min], [x_max, y_max]], or ``None`` if the cell is empty. """

        if len(self.elements) == 0:
            return None
        if not (self._bb_valid and
                all(ref._bb_valid for ref in self.get_dependencies(True))):
            bb = numpy.array(((1e300, 1e300), (-1e300, -1e300)))
            all_polygons = []
            for element in self.elements:
                if isinstance(element, PolygonSet):
                    all_polygons.extend(element.polygons)
                elif isinstance(element, CellReference) or isinstance(
                        element, CellArray):
                    element_bb = element.get_bounding_box()
                    if element_bb is not None:
                        bb[0, 0] = min(bb[0, 0], element_bb[0, 0])
                        bb[0, 1] = min(bb[0, 1], element_bb[0, 1])
                        bb[1, 0] = max(bb[1, 0], element_bb[1, 0])
                        bb[1, 1] = max(bb[1, 1], element_bb[1, 1])
            if len(all_polygons) > 0:
                all_points = numpy.concatenate(all_polygons).transpose()
                bb[0, 0] = min(bb[0, 0], all_points[0].min())
                bb[0, 1] = min(bb[0, 1], all_points[1].min())
                bb[1, 0] = max(bb[1, 0], all_points[0].max())
                bb[1, 1] = max(bb[1, 1], all_points[1].max())
            self._bb_valid = True
            _bounding_boxes[self] = bb
        return _bounding_boxes[self]