How do I patch or fill intersecting areas in one figure with several curves?

Question

I am trying to fill different section of a multi plot figure. This is my code:

function BFL

    a1=(-10):0.1:10;
    b=((1+a1).^2)./4;
    a2=1;
    B=[b,fliplr(b)];
    A=[a1, fliplr(a2)];

    fill(A,B, 'b');
    hold on;

    a=(-10):0.5:10;
    b=(-10):0.5:10;  

    hold on;

    a=1;
    b=(-10):0.1:10;
    plot(a,b);
    hold on;

    xlabel('parameter a');
    ylabel('parameter b');
    axis ([-10 10 -10 10]);

end