Joke answer: the bits that aren't in there. Real answer: Mainly the JIT engine, and the very complex rewriting algorithms that turn multi-threaded soul code in

Joke answer: the bits that aren't in there. Real answer: Mainly the JIT engine, and the very complex rewriting algorithms that turn multi-threaded soul code into a form that can actually be executed. We're certainly keen to open-source everything when possible, just been advised by our lawyers not to do that yet. C'est la vie.