- Implementations are written in Lisp all the way down. You can immediately jump to the definition of anything at any depth, and often edit or redefine to taste. (Example, I improved SBCL’s integer square root in my live image, and submitted a patch.) The only off-limits bits are a part of the runtime, like the garbage collector.

I'd add even the GC is accessible if you have the knowledge. Grammarly has a blog post that talks in part about manipulating parts of the GC to achieve higher through put by keeping heap objects (I think!) from reclamation.