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This is an easy one to begin with. The program simply runs by dividing the plane into three areas and then successively divides the three areas into three and so on – you can see the hierarchical tree explicitly and this is as fine an example of recursion as you can imagine. There are many variants of this process and many variants of Sierpinski’s tree. In his book the Fractal Geometry of Nature, Benoit Mandelbrot (1983) presents many diverse examples and if you search the web you will find many online demonstrations which enrich this application.