0 x = 0: v = 1: f = 0: c = 0
1 CLS : SCREEN 12
2 x = x + v
3 c = c + f
4 IF x = 1 THEN c = c + 1
5 IF c = 15 THEN c = c - 16
30 CIRCLE (340, 240), x, c: CIRCLE (340, 240), x + 1, 0: CIRCLE (340, 240), x - 1, 0
40 IF x = 240 THEN v = -1
50 IF x = 1 THEN v = 1
60 A$ = INKEY$
70 IF A$ = CHR$(32) THEN END
80 GOTO 2

