Picture your code. Picture all those functions on which there are no source-code dependencies. That might sound odd: if there are no source-code dependencies on a function then what is its purpose? Well, we must distinguish between compile-time dependencies and run-time dependencies. True, all entities at run-time must be called by other run-time entities: it's turtles all the way down (or at least all the way to freshly powered-up hardware and its firmware-initialized instruction pointer).