{
  # Defines a minimal `nixosConfigurations` output and verifies a known
  # value from the evaluated system configuration.
  description = "A minimal nixosConfigurations example";

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

  outputs =
    { nixpkgs, ... }:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};

      demoSystem = nixpkgs.lib.nixosSystem {
        inherit system;
        modules = [
          ./module.nix
          {
            boot.loader.grub = {
              enable = true;
              device = "/dev/null";
            };

            fileSystems."/" = {
              device = "tmpfs";
              fsType = "tmpfs";
            };

            system.stateVersion = "24.11";

            playground = {
              hostName = "demo-machine";
              motd = "hello from nixosConfigurations";
            };
          }
        ];
      };
    in
    {
      nixosConfigurations.demo = demoSystem;

      checks.${system}.configuration =
        pkgs.runCommand "nixos-configuration-check"
          {
            inherit (demoSystem.config.networking) hostName;
            motd = demoSystem.config.environment.etc."playground-motd".text;
          }
          ''
            if [ "$hostName" != "demo-machine" ]; then
              echo "unexpected host name: $hostName" >&2
              exit 1
            fi

            if [ "$motd" != "hello from nixosConfigurations" ]; then
              echo "unexpected motd: $motd" >&2
              exit 1
            fi

            echo ok > "$out"
          '';
    };
}