SCREEN 12: CLS : pi = 3.14159
LOCATE 12, 20: PRINT "press any key to start"
DO: LOOP WHILE INKEY$ = ""
CLS
FOR i = 1 TO 200
  PSET (i, 50 * SIN(4 * pi * i / 639) + 239), 1
  FOR j = 1 TO 2000: NEXT j
NEXT i
DO WHILE INKEY$=""
FOR i = 200 TO 839
   PSET (i, 50 * SIN(4 * pi * i / 639) + 239), 1
   PSET (i-639, 50 * SIN(4 * pi * i / 639) + 239), 1
   PRESET (i - 200, 50 * SIN(4 * pi * (i - 200) / 639) + 239)
   FOR j = 1 TO 2000: NEXT j
NEXT i
LOOP