The output of a pattern can feed back to a pattern at a lower level or even to the pattern itself, giving the human brain its powerful recursive ability. An element of a pattern can be a decision point based on another pattern. This is especially useful for lists that compose actions—for example, getting another tube of toothpaste if the current one is empty. These conditionals exist at every level. As anyone who has attempted to program a procedure on a computer knows, conditionals are vital to describing a course of action.
