In CityEngine, there is a built-in cube asset that many of us use frequently. There is not, however, a built-in circle.
So, I thought, why not truncate a square until it looks like a circle. I have 4 levels of truncation in the rule, where the corners of the polygon are cut off so that it doubles the number of sides and keeps the sides at equal lengths. It gets more and more circular past level 1, which is an octagon. The rule, Circle.cga, is attached.