{
  # Customizes an existing package with `overrideAttrs` and verifies that
  # the override changes the package's runtime behavior.
  description = "Customize an existing package with overrideAttrs";

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

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

      customizedHello = pkgs.hello.overrideAttrs (old: {
        pname = "hello-override";
        nativeBuildInputs = (old.nativeBuildInputs or [ ]) ++ [ pkgs.makeWrapper ];
        postInstall = (old.postInstall or "") + ''
          wrapProgram $out/bin/hello \
            --add-flags "--greeting=hello-from-overrideAttrs"
        '';
      });
    in
    {
      packages.${system}.default = customizedHello;

      apps.${system}.default = {
        type = "app";
        program = "${self.packages.${system}.default}/bin/hello";
        meta.description = "Run GNU hello with a greeting injected by overrideAttrs.";
      };

      checks.${system}.greeting = pkgs.runCommand "hello-override-check" { } ''
        output="$(${self.packages.${system}.default}/bin/hello)"

        if [ "$output" = "hello-from-overrideAttrs" ]; then
          echo ok > "$out"
        else
          echo "unexpected output: $output" >&2
          exit 1
        fi
      '';
    };
}