derivation {
  name = "filter";
  system = "@system@";
  builder = "@shell@";
  args = ["-e" "-x" (builtins.toFile "builder" "PATH=@testPath@; ln -s $input $out")];
  input =
    let filter = path: type:
      type != "symlink"
      && baseNameOf path != "foo"
      && !((import ./lang/lib.nix).hasSuffix ".bak" (baseNameOf path));
    in builtins.filterSource filter ./test-tmp/filterin;
}