let
    # [CommentDb: 2]
    nixpkgsTarball = {
        "18.09"  = fetchTarball https://nixos.org/channels/nixos-18.09/nixexprs.tar.xz;
        unstable = fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz;
        master   = fetchTarball https://github.com/NixOS/nixpkgs/archive/master.tar.gz;
    };
    tarball_18_09 = fetchTarball https://nixos.org/channels/nixos-18.09/nixexprs.tar.xz;

    tarball_unstable = fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz;

    # Github master
    tarball_master  = fetchTarball https://github.com/NixOS/nixpkgs/archive/master.tar.gz;
in

...
...
...

        nixpkgs.config = {
            allowUnfree = true;
            packageOverrides = pkgs: {
                unstable = import tarball_unstable {
                    config = config.nixpkgs.config;
                };


                # This doesn't work
                archived = builtins.listToAttrs
                (
                    lib.attrsets.mapAttrsToList
                    (
                        name: value:
                        {
                            name = name;
                            value = import (nixpkgsTarball.${value}){
                                config = config.nixpkgs.config;
                            };
                        }
                    )
                    nixpkgsTarball
                );
            };
        };