Idiomatic Lisp and Idiomatic Scheme

I guess I agree with Brian Marick's code reading style more than Richard Gabriel's. Rather than using an optional private parameter in a public function, though, I prefer to use the "named" let, with an accumulator there, for tail recursive functions.

Is it idiomatic? It's fairly common in Scheme. I am not sure why it has not been adopted more widely into Common Lisp.

;; Richard Gabriel's Idiomatic Common Lisp
(defun fact (n)
  (labels ((f (n acc)
	      (if (<= n 1) acc (f (- n 1) (* n acc)))))
    (f n 1)))

;; My (Idiomatic?) Scheme
(define (fact n)
  (let loop ((n n) (acc 1))
       (if (<= n 1)
	   acc
	 (loop (- n 1) (* n acc)))))

;; "Old School" Common Lisp
(defun fact(n &optional (acc 1))
  (if (<= n 1)
      acc
    (fact (- n 1) (* n acc)))