Nixpkgs is a vast collection of packages, yet sometimes we need to add our own modification or additional packages to it.
The intended way to do so are overlays which are composable extensions of package sets.

Still, while documentation on overlays exists, the details behind package sets such as mkScope, mkExtensible, etc. remain relatively elusive.

In this talk I will shed some light on this increasingly consistent architecture of nixpkgs, point out ways to automate and where automation fails.