I'm particularly fond of this pattern when you can implement the high level API that you want outside the library, which ensures that your low level API is suff

I'm particularly fond of this pattern when you can implement the high level API that you want outside the library, which ensures that your low level API is sufficiently flexible and means you're dogfooding your own stuff as a user. It's far too easy to get used to the internal side of a tool you're building, and forget how people actually use it.