(in-package :cl-user)

(defun mul (a b)
  (if (< a 0)
      (setq a (- a) b (- b)))
  (let ((r 0))
    (loop until (zerop a)
          do
          (unless (zerop (logand 1 a))
            (incf r b))
          (setq a (ash a -1)
                b (+ b b)))
    r))

#+PLZ
(defun * (a b)
  (when (< a 0)
    (setq a (- a)
          b (- b)))
  (let ((r 0))
    (while a
      (when (logand 1 a)
        (incf r b))
      (setq a (asr a)
            b (+ b b)))))