Recall that the description of the program next says that every time it is applied, it redraws the picture on a canvas according to the common traffic rules. For another example, take a look at the Scheme function random . It consumes a natural number n and produces a number between 0 and n -1. Again, to achieve this effect, the implementor of random needed to equip the function with some memory. They represent each service that the program is to support with a rectangular box.
Memory is represented with circles.