@NotNull
  static VirtualFileEnumeration getFileEnumerationUnderRoots(@NotNull Collection<? extends VirtualFile> roots) {
    IntSet result = new IntOpenHashSet();
    for (VirtualFile file : roots) {
      if (file instanceof VirtualFileWithId) {
        int[] children = VirtualFileManager.getInstance().listAllChildIds(((VirtualFileWithId)file).getId());
        result.addAll(IntList.of(children));
      }
    }

    return new MyVirtualFileEnumeration(result);
  }
@NotNull
  static VirtualFileEnumeration getFileEnumerationUnderRoots(@NotNull Collection<? extends VirtualFile> roots)