prog style

prog.lisp

;; a la MACLISP
(DEFUN FACTORS (N)
  (PROG (FACS SQRTN D)
        (SETQ FACS () )
        (SETQ D 2)
        (GO L1)
    L2  (SETQ N (FLOOR N D))
        (SETQ FACS (CONS D FACS))
    L1  (SETQ SQRTN (ISQRT N))
    L   (COND ((> D SQRTN) (GO X)))
        (COND ((ZEROP (MOD N D)) (GO L2)))
        (SETQ D (1+ D))
        (GO L)
    X   (COND ((= 1 N) (RETURN FACS)))
        (SETQ FACS (CONS N FACS))
        (RETURN FACS)))

;; a la Common Lisp
(defun factors (n)
  (prog ((facs '())
         (sqrtn (isqrt n))
         (d 2))
        (declare (integer sqrtn d)
                 (list facs))
        (go L)
    L1  (setq n (floor n d))
        (push d facs)
        (setq sqrtn (isqrt n))
    L   (when (> d sqrtn) (go X))
        (when (zerop (mod n d)) (go L1))
        (incf d)
        (go L)
    X   (when (= 1 n) (return facs))
        (push n facs)
        (return facs)))

(factors 9999999999)
→ (9091 271 41 11 3 3)