Emacs Lisp range function (and factorial calculation)

range.el

(cl-defun range (end &optional (start 0) (step 1) reverse)
  "Returns a list of numbers from START to END incremented by STEP"
  (let ((i start) (res (list start)))
    (while (< i end)
      (setq i (+ i step))
      (if reverse
	  (setq res (cons i res))
	(setcdr (last res) (cons i nil))))
    res))

(defun ! (n)
  "Factorial calculation for N"
  (if (and (numberp n) (>= n 0))
      (if (eq n 0) (setq n 1))
    (error "N has to be a non-negative integer"))
  (apply '* (range n 1)))