{ lib }: { root, subdir ? null }:
let
  fs = lib.fileset;
  sourceFiles = fs.difference
    (fs.gitTracked root)
    (fs.fileFilter (file: file.hasExt "nix") root);
  finalSourceFiles =
    if subdir == null then
      sourceFiles
    else
      fs.intersection sourceFiles subdir;
  finalRoot = if subdir == null then root else subdir;
in
  fs.toSource { root = finalRoot; fileset = finalSourceFiles; }