I just have to say this: the NixOS module system is a brilliant piece of work. Once again, it works so well that it feels like magic.
Explanation: I'm splitting my large nixos and home-manager config files into logical units. To do this, you need to use the module system via the imports utility. If I declare a simple text file in an import, and re-declare its content in a subsequent import, the module system knows not to overwrite, but append it. So useful!
And the same applies to attribute sets, and probably other data types (lists?) too! A.M.A.Z.I.N.G.