The syntax for parametric surfaces is very different. Here's a sort of conical spiral that would be very difficult to specify without using parametric equations. This is the same surface as last time, but in this case I've declared two of the equations beforehand. I get exactly the same surface. In POV-Ray 3.5 it is possible to perform variable substitution here.
Higher values cause the parse time to become rather long.
In this case, there's no speed gain from precomputing y, but there's not much of a penalty either. I get the distinct impression that things like pigment functions aren't really supported with parametric surfaces. This is the Astroidal Ellipse. The surface goes in where an ordinary ellipse goes out. This surface is called the Bohemian Dome.