10 CLS:X=5:Y=5:DATA1,1,39,1,-1,42,1,-1,37:FORB=1TO3:READV,W,I
20 PLOTX,Y,42:X=X+V:Y=Y+W:IFX=2ORX=20THENV=V*-1ELSEIFY=5ORY=23THENW=W*-1
30 I=I-1:IFITHEN20ELSENEXT

