function double_spiral()
    t=0:pi/20:5.5*pi;
    x1=t.*cos(t);
    y1=t.*sin(t);
    clf;
    plot(x1,y1);
    hold on;

    t=0:pi/20:5.5*pi;
    x2=t.*cos(t) - 50;
    y2=t.*sin(t);
    plot(-x2,y2);

    hold on;

    plot(0:50,y1(end))