Illustrates tail-recursion

add.lisp

(defun add (n)
    (if (= n 0)
        n
        (+ n (add (- n 1)))
    )
)

(defun add_optimized (n)
    (defun iter (x acc)
        (if (= x 0)
            acc
            (iter (- x 1) (+ acc x))
        )
    )

    (iter n 0)
)

(format t "~a%" (add 3))
(format t "~a%" (add_optimized 3))