anchor.diff:      (defun add-animation (app)
         (let ((elapsed 0)
   	    (time false))
  -	(with-slots (timefactor pause toggle-pause init reset update-anim sail update-tilt render animate controls accelfn sail-position scene) app
  +	(with-slots (timefactor pause toggle-pause init reset update-anim sail update-tilt render animate controls accelfn sail-position scene velocity-arrow) app
   	  (with-slots (position velocity) sail-position
   	    (setf
   	     ;; Sail position & velocity
   	     sail-position (new (3fn *object3-d))
   	     position (new (3fn *vector3 0 0 0))
   	     velocity (new (3fn *vector3 0 0 0))
  +	     velocity-arrow (new (3fn *arrow-helper (incident) (origin) 0 0x0000ff))
   	     timefactor 10
   	     pause false
   	     toggle-pause
  
positive.code: 			(funcall render))))
  		 ;; Attach sail object to sail-position & put in scene
  		 ((@ sail-position add) sail)
  		 ((@ scene add) sail-position)))
  	     accelfn
  	     (lambda () (new (3fn *vector3 0 0 0)))
  	     update-anim
  	     (lambda ()
  	       (let* ((accel-v (funcall accelfn))
  		      (now ((@ (new (*date)) get-time)))
  		      (dt (/ (* timefactor (- now (or time now))) 1000)))
  		 (setf time now)
  		 (incf elapsed dt)
  		 ((@ velocity add) ((@ ((@ accel-v clone)) multiply-scalar) dt))
  		 ((@ position add) ((@ ((@ velocity clone)) multiply-scalar) dt))
  		 ;;((@ sail position copy) position)
  		 (setf (@ ($ "#accel") 0 inner-h-t-m-l) ((@ ((@ *math floor10) (* ((@ accel-v length)) 1000) -4) to-string)))
  		 (setf (@ ($ "#speed") 0 inner-h-t-m-l) ((@ ((@ *math floor10) (* ((@ velocity length)) 1000) -2) to-string)))
  		 (setf (@ ($ "#distance") 0 inner-h-t-m-l) ((@ ((@ *math floor10) ((@ position length)) -3) to-string)))
  
negative.code: 	((@ scene add) reflect-arrow))
        app)
      
      (defun add-target (app &key (imin -90) (imax 90))
        "Add a big red randomly positioned target"
        (with-slots (scene target) app
  	(let ((inc2 (* (+ (* 2 (+ imin (* 5 (random (1+ (/ (- imax imin) 2 5)))))))
  		       (/ pi 180)))
  	      (rot (* 5 (random (/ 360 5)) (/ pi 180)))
  	      (dist (+ 20 (random 80)))
  	      (rad 5)
  