This `draw' package provides libraries for modeling in a visual world. It
consists of two sets of classes: 

 - World and Posn 

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

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


The _World_ class has the following class interface: 

     // create the visual aspect 
     boolean start(int width, int height)

     // tear down the canvas and shut down the program
     boolean stop()

     // draw a circle at p, paint circle c
     boolean drawCircle(Posn p, int r, Color c)

     // draw a solid disk at p, fill with color c
     boolean drawDisk(Posn p, int r, Color c)

     // draw a width x height rectangle at p, fill with color c
     boolean drawRect(Posn p, int width, int height, Color c)

     // draw a line from p0 to p1, use color c
     boolean drawLine(Posn p0, Posn p1, Color c)

     // draw a string at position p
     boolean drawString(Posn p, String s)

     // clear a circle at p, paint circle c
     boolean clearCircle(Posn p, int r, Color c)

     // clear a solid disk at p, fill with color c
     boolean clearDisk(Posn p, int r, Color c)

     // clear a width x height rectangle at p, fill with color c
     boolean clearRect(Posn p, int width, int height, Color c)

     // clear a line from p0 to p1, use color c
     boolean clearLine(Posn p0, Posn p1, Color c)

     // wait for s seconds (roughly)
     boolean sleepForAWhile(int s)

     // start the clock and make this world the current one
     boolean bigBang(double s) 

     // process a tick of the clock in this world
     World onTick() 

     // process a keystroke event in this world
     World onKeyEvent(String ke) 

     // stop this world's clock
     World endOfWorld() 

     // draw this world
     boolean draw() 

     // erase this world
     boolean erase() 

     // view the last World that onTick or onKeyEvent or bigBang created
     World lastWorld()

_Posn_ is the usual class: it combines two integers, x and y, and makes the
 available via the selectors Posn.x and Posn.y.