private static @Nullable Set<VirtualFile> addFile(@Nullable Set<VirtualFile> set, @Nullable VirtualFile file) {
    if (file == null) return set;
    if (set == null) set = new LinkedHashSet<>();
    set.add(file);
    return set;
  }
private static @Nullable Set<VirtualFile> addFile(@Nullable Set<VirtualFile> set, @Nullable VirtualFile file)