buffer = 0

open window 640, 512

setrgb 0, 0, 0, 0

clear window

for i = 1 to 1000

  buffer = 1 - buffer

  setdrawbuf buffer

  setrgb 1, mod(i, 256),  mod(i, 256),   mod(i, 256)
  setrgb 0, 0, 0, 0

  print mod(i, 200)

  clear window

  circle 320, 256, 1 + mod(i, 200)

  setdispbuf buffer

next i