[Closed] Zenon stadium paradox. What am I doing wrong?

You all know the Zenon stadium paradox (move half the distance at each step, and the runner never reachs the goal)…
I have tried to reproduce it using a position_script controller and… it really happen to me!
I don’t know why my teapot reachs quickly a distance and then stays at this distance and does not come any closer. What am I doing wrong?
(It’s just an exercise of using own last position in a position_script controller)
The code:

(
   -- Set Scene ----
   delete objects
   theRunner = teapot radius:10 pos:[0,0,0]
   theGoal = sphere radius:10 pos:[200,0,0]
   
   with animate on
   (
      for f = 10 to 100 by 10 do
      (
         with animate on at time f
         (
            angle = 36 * f/10
            theGoal.pos = [200 * cos angle, 200 * sin angle, 0]
         )
      )
   )
   ---------------
   
   pos0 = theRunner.pos
   sNode = nodeMonitor node:theRunner
   
   ctrl1 = Position_Script()
   ctrl1.addNode "goal" theGoal
   ctrl1.AddObject "sNode" sNode
   ctrl1.AddConstant "pos0" pos0
   ctrl1.AddConstant "lastPos" pos0
   
   theRunner.position.controller = ctrl1
   
   str1 = 
   ("
      if (F != 0) then
      (
         pos = (lastPos + goal.Pos) / 2.0
         sNode.node.position.Controller.SetConstant \"lastPos\" pos
         --print (distance pos goal.Pos)
         pos
      )
      else
      (
         sNode.node.position.Controller.SetConstant \"lastPos\" pos0
         pos0
      )
   ")
   
   ctrl1.script = str1
)

Edit: OK. I know the answer.