It might also be worth mentioning that the rhinoscriptsyntax Python package is located here:
C:\Users\USERNAME\AppData\Roaming\McNeel\Rhinoceros\6.0\Plug-ins\IronPython (814d908a-e25c-493d-97e9-ee3861957f49)\settings\lib\rhinoscript
Where you can browse through the .py files and learn how RhinoCommon has been implemented:
I like to use Sublime Text for this. Where I add the rhinoscript folder (under Project > Add Folder to Project), which allows you to search across all the .py files at once for a function:
Then double click the search results in the editor, and hey presto:
As you can see here, most of the SplitBrep() code is related to guid/Rhino document stuff, while it’s only line 2383 that is actually doing any RhinoCommon geometry business. If you’re in GHPython, I find it simpler/better/faster to go directly with RhinoCommon when computing geometry stuff.