planet/system generation question

[wilcoxon]

I'm trying to dig into the gritty details of system generation and it looks like I can only go so far based off of the data files.

If an entry in systemtypes.txt has "Size := Any", "Composition := Any", and "Atmosphere := Any", how is the planet generated? I can see two main possibilities:

1) One attribute is generated randomly, then the others are generated randomly from a list of valid values based on the previously generated ones. If this is correct, what is the order of the attribute generation?
2) A planet is chosen at random from secttype.txt.

Anybody have any idea which (if either) of these is correct?

Also, is it possible to have a Ringworld or Sphereworld be both built and "natural" (build by long-dead civilization)?