You don't need to know either (or any other language for that matter) to bang out a project of some kind via Arc.
But to hang with the language theory discussions here you are going to want to know everything you can about functional programming, and especially PLT's implementation of scheme, because that is what Arc runs on.
Now you would expect every function that references car to use your new version. If you can't trust basic functions such as car, data-flow analysis becomes very hard. I'm not an expert and I have only a vague idea of what data-flow analysis is, but I think this could be the problem. This is only what I think, so I could be completely wrong.