Ultra-SiMPLE
size = 150
x = xpixels()/2
y = ypixels()/2
dx = 5
Do
   cls
   circle (x, y, size)
   delay (25)
   x = x + dx
   If (x < size) dx = -dx
   If (x > xpixels()-size) dx = -dx
   read quitkey (27)
Loop