Just curious, how do you open a rule package and use it in CityEngine? I managed to download baselayer.zip containing the portland example with some rules as cga and some as rpk. I want to see how the rules are created.
RPKs are basically zipped rule files including assets.
.rpk is actually a 7-zip file, so what you downloaded is a kind of zipped zip file.
If you have the .rpk file, you can use it directly in CityEngine and assign it to a shape.
If you want to look into the .rpk file, you can extract the contents as from a standard zip file. But it may be the case that the CGA code in text is not available in the rpk. It can be that only the CGB file is in there, which is a binary file that you can't open and look at.
Please let me know if there's more questions.