Example 4 (Figures 12 and 13)

> z := 1/3 * sqrt( 36-9*x^2-4*y^2 );

> plot3d( z, x=-2..2, y=-3..3, style=contour, contours=[0, 1, 1.5, 1.75, 1.999], orientation=[-90,0], title="Figure 12" );

> plot3d( z, x=-2..2, y=-3..3, style=patchcontour, contours=[0, 1, 1.5, 1.75, 1.999] );

>

> z := y^2 - x^2;

> plot3d( z, x=-3..3, y=-3..3, style=contour, contours=[$-4..4], orientation=[-90,0], title="Figure 13" );

> plot3d( z, x=-3..3, y=-3..3, style=patchcontour, contours=[$-4..4] );

>