{
  # Packages a selected local source tree so the derivation only sees the
  # files that the example deliberately includes.
  description = "Package a local source tree with fileset";

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

  outputs =
    { self, nixpkgs, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs { inherit system; };
      sourceFiles = pkgs.lib.fileset.unions [
        ./src/message.txt
        ./src/run.sh
      ];
      selectedSource = pkgs.lib.fileset.toSource {
        root = ./.;
        fileset = sourceFiles;
      };
    in
    {
      packages.${system}.default = pkgs.stdenv.mkDerivation {
        pname = "source-greet";
        version = "0.1.0";
        src = selectedSource;
        dontBuild = true;
        dontUnpack = true;

        installPhase = ''
          mkdir -p $out/bin $out/share/source-greet
          cp $src/src/message.txt $out/share/source-greet/message.txt
          cp $src/src/run.sh $out/bin/source-greet
          chmod +x $out/bin/source-greet

          substituteInPlace $out/bin/source-greet \
            --replace-fail "@bash@" "${pkgs.bash}/bin/bash" \
            --replace-fail "@message@" "$out/share/source-greet/message.txt"
        '';
      };

      apps.${system}.default = {
        type = "app";
        program = "${self.packages.${system}.default}/bin/source-greet";
        meta.description = "Run the package built from the selected local source tree.";
      };

      checks.${system}.selected-source = pkgs.runCommand "selected-source-check" { } ''
        [ ! -e ${selectedSource}/ignored.txt ]
        output="$(${self.packages.${system}.default}/bin/source-greet)"

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