This `draw' package provides libraries for modeling in a visual world:


		     +----------+     +----------+
		     | World    |  +->| Canvas   |
		     +----------+  |  +----------+
                     | Canvas c |--+  +----------+
		     +----------+     | draw     |
		     +----------+     | clear    |
				      +----------+

 - abstract class World 

 - class Canvas 

 - class Posn 

		      +----------+
		      | Posn     |
		      +----------+
		      | int x    |
		      | int y    |
		      +----------+

 - Color with five subclasses: 
   + Blue 
   + Green 
   + Red 
   + White 
   + Yellow 

                              +-------+
                              | Color |
                              +-------+
                                  |
                                 / \
                                 ---
                                  |
             ------------------------------------------
             |          |         |         |         |     
         +-------+  +-------+ +-------+ +-------+ +-------+  
         | Blue  |  | Green | | Red   | | White | | Yellow| 
         +-------+  +-------+ +-------+ +-------+ +-------+