function menger(level,lit)
    % Fractal cube, the Menger sponge.
    % menger(level) generates 20^level cubes.
    % menger(level,true) is illuminated. 
    % menger(level,false) is not illuminated, so it is faster.
    
    if nargin < 1, level = 3; end
    if nargin < 2, lit = true; end
    
    V = [-3 -3 -3; -3 -3 3; -3 3 -3; -3 3 3;
          3 -3 -3;  3 -3 3;  3 3 -3;  3 3 3]; 
    init_fig(lit)
    sponge(V,level,lit)
    
    % --------------------------------------------------------
    
    function sponge(v,level,lit)
        if level > 0
            v = v/3;
            for x = [-2 0 2]
                for y = [-2 0 2]
                    for z = [-2 0 2]
                        if nnz([x y z]) > 1
                            sponge(v+[x y z],level-1,lit)
                        end 
                    end
                end
            end
        else
            cube(v,lit)
        end
    end

    function cube(v,lit)
        f = [ 1 5 7 3
              3 7 8 4
              1 3 4 2
              2 4 8 6
              1 2 6 5
              5 6 8 7];
        gold = [1 .85 .60];
        p = patch(Vertices = v, ...
                Faces = f, ...
                FaceColor = gold, ...
                LineWidth = 0.5); 
        if lit
            golden = [0.7,0.5,0.3,1.0,0.2];
            material(p,golden)
        end   
    end

    function init_fig(lit)  
        axis(4*[-1 1 -1 1 -1 1])
        axis equal off vis3d
        rotate3d on
        view(60,15) 
        if lit
            camlight(-45,10);
            % set(gcf,'windowbuttonmotionfcn',@wbmf);
        end
    end

    function wbmf(~,~)
        a = gca;
        l = findobj(a,'type','light');
        camlight(l,-45,10);
    end         
 end