{ stdenv, lib }:
let 
  fs = lib.fileset;
  sourceFiles = fs.unions [
    ./hello.txt
    ./world.txt
    ./build.sh
    (fs.fileFilter
      (file: file.hasExt "c" || file.hasExt "h")
      ./src
    )
  ];
in

fs.trace sourceFiles

stdenv.mkDerivation {
  name = "fileset";
  src = fs.toSource {
    root = ./.;
    fileset = sourceFiles;
  };
  postInstall = ''
    cp -vr . $out
  '';
}