Functions that accept two args (conventionally: final, prev) and return a set of [[package]]s.

The `prev` arguments holds the set of packages of the "parent" overlay while the `final` argument holds the end result of **all** overlays applied. This means that overlays depend on each other. It is unclear how we can tell on which overlay an overlay depends.

We can use it to override existing packages or add new packages.

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https://nixos.wiki/wiki/Overlays