; 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)