[Zope-dev] Where does Acquisition.so come from?

I'm trying to get a fuller understanding of the way Zope works and the time has come to see what is going on inside Acquisition.aq_get(), but this is inside a compiled object, and I can't find anything resembling a source file in my Zope-2.3.3-src release. Am I just mistaken about which relese I downloaded? Where does Acquisition.so come from? Thanks in advance. John