A second version of the mutation operators introduced ``smooth'' mutations, which have better probabilities of producing small changes in the structure. The original mutation was simply ``replace a randomly selected brick with a random single brick''. The smooth mutations are of four kinds:
A randomly selected brick is enlarged, either to the left or right, to the nearest valid brick size.
A randomly selected brick is shrunk, either from the left or the right, to the next smaller valid brick size.
A randomly selected brick shifts position, either to the left or right, by one knob. Any descendant bricks will either be ``pushed'' along in the same direction, or remain in their original positions.
A brick is randomly selected. A random knob is chosen within that brick, above or below, and a new random brick is added at that point.