static L<FileWithSize> findDirsWithSizes(File dir) {
  new L<FileWithSize> out;
  findDirsWithSizes(dir, out);
  ret out;
}

svoid findDirsWithSizes(File dir, L<FileWithSize> out) {
  long clusterSize = assumedClusterSize();
  long n = 0;
  new L<FileWithSize> l2;
  for (File f : listFiles(dir))
    if (f.isDirectory())
      findDirsWithSizes(f, l2);
    else
      n += roundUpTo(clusterSize, f.length());
  for (FileWithSize f : l2) n += f.size;
  out.add(FileWithSize(dir, n));
  out.addAll(l2);
}