http://www.p01.org/releases/#128b_raytraced_checkboard を xyzzyに移植

checkboard.lisp

;;; http://www.p01.org/releases/#128b_raytraced_checkboard

(defun draw-checkboard (n &optional (s *standard-output*))
  (do* ((k 64) (i k))
      ((= (decf i (/ k)) 0))
    (let ((j (/ k i)))
      (princ (char "p.\n"
		   (if (and (integerp i) (evenp i))
		       2
		     (logand 1
			     (logxor (floor j)
				     (floor (+ (* (mod i 2) j) (- j) (/ n k)))))))
	     s))))

(defun checkboard ()
  (interactive)
  (let ((n 0) (buf (get-buffer-create "*check*")))
    (labels ((frame ()
	       (if (eq *this-command* 'quit)
		   #1=(progn
			(stop-timer #'frame)
			(delete-buffer buf)
			(refresh-screen))
		 (handler-case
		     (progn
		       (set-buffer buf)
		       (erase-buffer buf)
		       (with-output-to-buffer (buf)
			 (draw-checkboard (incf n 7)))
		       (refresh-screen))
		   (quit (c) #1#)))))
      (start-timer 1/30 #'frame))))

; press C-g to stop

どうもー　修正しました。