The four rules in the example above illustrate the basic ideas of goal-directed rule-based expert systems. However, in general of course there would be hundreds or thousands of rules in the system, and each rule would be considerably more complex. Rules will almost certainly involve patterns with variables (e.g., age(Patient, X)), rather than just simple unstructured propositions (e.g., age_fred_23)) and will usually have certainty factors attached to them (described later).
However, although real systems will be much more complex that the above, many use the same basic reasoning and explanation procedures. One early system which used this basic approach is MYCIN, a system for diagnosing blood disorders. We'll describe this system in a little more detail later.