3.1.2.1 Example

(defrule move-train :forward

          :context train

          (train ?train position ?train-pos)

          (signal ?signal position ?signal-pos 

           colour green)

          (test (= ?signal-pos (1+ ?train-pos)))

-->

  ((format t "~%Train moving to position ~s" 

              ?signal-pos))

  (assert (signal ?signal colour red))

  (assert (train ?train position ?signal-pos)))

specifies that if there is a train with a green signal directly in front then the train may move on and the signal changes to red.