; animate
(
  (clear) 
  (def t 
    (sin 
      (div 
        (time) 100))) 
  (def pos 
    (add 200 30 
      (mul 30 t))) 
  (defn square 
    (a) 
    (rect a a a a)) 
  (stroke 
    (square pos) 1 "red") 
  ; set false to stop 
  (animate true))