Inheritance and
part-of relationship
On the object level it is possible now to use a circle just as
we would use a point, because a circle is-a point. For
example, a circle object and set its center point coordinates
can be defined as follows:
  Circle acircle
  acircle.setX(1)        /* Inherited from Point */
  acircle.setY(2)
  acircle.setRadius(3)   /* Added by Circle */