(define (factorial n)
  (if (zero? n)
      1
      (* n (factorial (- n 1)))))


;;; ANF

(define (factorial n)
  (let ((r₁ (zero? n)))
    (if r₁
        1
        (let ((r₂ (- n 1))
              (r₃ (factorial r₂))
              (r₄ (* n r₃)))
          r₄))))