{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-20.09";
  inputs.nixpkgs_unstable.url = "github:NixOS/nixpkgs/nixos-unstable";

  outputs = { self, nixpkgs, nixpkgs_unstable }: {

    nixosConfigurations = let
      mkConfig = { hostName, module ? (./. + "/machines/${hostName}.nix") }: {
        name = hostName;
        value = nixpkgs.lib.nixosSystem {
          system = "x86_64-linux";
          modules =
            [
              module
              ({config, pkgs, ...}: {
                # Let 'nixos-version --json' know about the Git revision
                # of this flake.
                system.configurationRevision =
                  if self ? rev then self.rev else "DIRTY";

                # Add unstable packages.
                nixpkgs.overlays = [
                  (final: prev: { unstable = nixpkgs_unstable.legacyPackages."x86_64-linux"; })
                ];
              })
            ];
        };
      };
      myMachines = [
        { hostName = "pincer"; module = ./machines/x1-pincer.nix; }
        { hostName = "dragon"; }
      ];
    in builtins.listToAttrs (map mkConfig myMachines);

  };
}