Exercise 2.3: Implement a representation for
rectangles in a plane.  (Hint: You may want to make use of Exercise 2.2.)
In terms of your constructors and selectors, create procedures that compute the
perimeter and the area of a given rectangle.  Now implement a different
representation for rectangles.  Can you design your system with suitable
abstraction barriers, so that the same perimeter and area procedures will work
using either representation?