Example 5 (Figure 14)

> z := x*y;

> plot3d( z, x=-3..3, y=-3..3, style=contour, contours=[-4,-1,0,1,4], orientation=[-90,0], title="Figure 14" );

> plot3d( z, x=-3..3, y=-3..3, style=patchcontour, contours=[-4,-1,0,1,4] );

>