RosettaCodeData/Task/Ackermann-function/CLIPS/ackermann-function-1.clips

(deffunction ackerman
  (?m ?n)
  (if (= 0 ?m)
    then (+ ?n 1)
    else (if (= 0 ?n)
      then (ackerman (- ?m 1) 1)
      else (ackerman (- ?m 1) (ackerman ?m (- ?n 1)))
    )
  )
)