static Tree<FileWithSize> findDirsWithSizes_tree(File dir) {
  long clusterSize = assumedClusterSize();
  long n = 0;
  new L<Tree<FileWithSize>> l2;
  for (File f : listFiles(dir))
    if (f.isDirectory())
      l2.add(findDirsWithSizes_tree(f));
    else
      n += roundUpTo(clusterSize, f.length());
  for (Tree<FileWithSize> f : l2) n += f.a.size;
  ret Tree(FileWithSize(dir, n), l2);
}