'Tuple defining the default ``bounding_box``.
``((y_low, y_high), (x_low, x_high))``'

@property
def bounding_box(self):

    dr = (self.r_in + self.width)
    return (((self.y_0 - dr), (self.y_0 + dr)), ((self.x_0 - dr), (self.x_0 + dr)))
