anchor.diff:        ((@ reflection-arrow set-direction) reflect)
         ((@ tangential-arrow set-length)
          (if (> ((@ tangential length)) 0) 10 0)))
  -    
  +
       ))
  
positive.code: 
      (defun sail-parts-visible (bool)
        "Change visibility of sail parts"
        (dolist (p (@ sail children))
  	(if (@ p obj) (setf (@ p obj visible) bool))))
  
      (defun render ()
        ;; Make sail parts invisible
  
negative.code: 	    (setf (@ r visible) false)))
        (arrows-visible false)
        ;; Update cube cameras
        (when sails
  	((@ sails mcam update-cube-map) renderer scene))
        (when vanes
  	(dolist (vane vanes)
  	  ((@ vane mcam update-cube-map) renderer scene)))
        ;; Reset visibility
        (sail-parts-visible true)
        (if projection (setf (@ projection visible) true))
  