static long guessClusteredDirSize(File dir, O... _) {
  optPar O pred; // F1<File, Bool>
  long n = 0;
  long clusterSize = assumedClusterSize();
  for (File f : findAllFiles_noDirs(dir))
    if (pred == null || isTrue_callF(pred, f))
      n += roundUpTo(clusterSize, f.length());
  ret n;
}

static long guessClusteredDirSize() {
  ret guessClusteredDirSize(programDir());
}