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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 reference. 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 not isinstance(self.ref_cell, Cell):
            return None
        if (self.rotation is None and self.magnification is None and
                self.x_reflection is None):
            key = self
        else:
            key = (self.ref_cell, self.rotation, self.magnification,
                   self.x_reflection)
        deps = self.ref_cell.get_dependencies(True)
        if not (self.ref_cell._bb_valid and
                all(ref._bb_valid for ref in deps) and key in _bounding_boxes):
            for ref in deps:
                ref.get_bounding_box()
            self.ref_cell.get_bounding_box()
            tmp = self.origin
            self.origin = None
            polygons = self.get_polygons()
            self.origin = tmp
            if len(polygons) == 0:
                bb = None
            else:
                all_points = numpy.concatenate(polygons).transpose()
                bb = numpy.array(((all_points[0].min(), all_points[1].min()),
                                  (all_points[0].max(), all_points[1].max())))
            _bounding_boxes[key] = bb
        else:
            bb = _bounding_boxes[key]
        if self.origin is None or bb is None:
            return bb
        else:
            return bb + numpy.array(((self.origin[0], self.origin[1]),
                                     (self.origin[0], self.origin[1])))