{
  # Exposes a `lib` attrset for pure helper values that other outputs can reuse.
  description = "Expose reusable helpers through lib";

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

  outputs =
    { self, nixpkgs, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs { inherit system; };

      playgroundLib = rec {
        rolloutPlan = [
          {
            service = "api";
            wave = 1;
            owner = "team-edge";
          }
          {
            service = "worker";
            wave = 2;
            owner = "team-batch";
          }
        ];

        ownerByService = builtins.listToAttrs (
          map (step: {
            name = step.service;
            value = step.owner;
          }) rolloutPlan
        );

        renderStep = step: "wave ${toString step.wave}: ${step.service} owned by ${step.owner}";

        renderPlan = builtins.concatStringsSep "\n" (map renderStep rolloutPlan);
      };
    in
    {
      lib = playgroundLib;

      packages.${system}.default = pkgs.writeShellApplication {
        name = "show-lib-plan";
        text = ''
          cat <<'EOF'
          ${self.lib.renderPlan}
          EOF
        '';
      };

      checks.${system}.lib-output =
        pkgs.runCommand "lib-output-check"
          {
            apiOwner = self.lib.ownerByService.api;
            workerLine = self.lib.renderStep (builtins.elemAt self.lib.rolloutPlan 1);
          }
          ''
            if [ "$apiOwner" != "team-edge" ]; then
              echo "unexpected api owner: $apiOwner" >&2
              exit 1
            fi

            if [ "$workerLine" != "wave 2: worker owned by team-batch" ]; then
              echo "unexpected worker line: $workerLine" >&2
              exit 1
            fi

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