static void printOccTree(OccTree t) {
  print("[" + t.count + "]");
  printOccTree_sub(t, "  ");
}

static void printOccTree_sub(OccTree t, S indent) {
  for (O e : keys(t.followUp)) {
    OccTree t2 = t.followUp(e);
    print(indent + "[" + t2.count + "] " + e);
    printOccTree_sub(t2, indent + "  ");
  }
}