Feel the math -- Surface

Have you ever troubles trying to imaging how a surface looks like? With OpenSCAD and 3D printer it is easy! You can write an equation z=f(x,y), print the result and actually feel the surface. Sample stl-files are for the equations:
z(x,y)=-.015(xx+yy);
z(x,y)=3(sin(10x) +sin(10y));
z(x,y)=.01(x+y)(x+y);
z(x,y)=.001xxx+.02y*y;
You you can use the file as a library for you model.