“This kind of recursive definition with a changing parameter is useful for producing anything that has a self-similar structure. A picture that contains a picture of itself is an example of a recursive, self-similar structure; such structures are commonly known as fractals. In the real world, self-similar structures don’t go on forever: for instance, each branch of a tree looks a lot like a smaller tree, and each of these smaller branches has branches that look like still smaller trees. This recursion goes on for several levels, but eventually the branches are so small that they do not have branches of their own.”