void Stick::mainloop(unsigned int x){
length=0;
for(;is_run();delay_fps(100)){
length=length+1;
if (keystate(VK_SPACE)){
cleardevice();
setlinestyle(SOLID_LINE,0,5);
line(x,300,x,300-length);
}
else break;
}
怎样只清掉棍子，而不cleardevice整个屏幕

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