TwoLevelHints* Binary::two_level_hints() {
  return const_cast<TwoLevelHints*>(static_cast<const Binary*>(this)->two_level_hints());
}
TwoLevelHints* Binary::two_level_hints() {
  return const_cast<TwoLevelHints*>(static_cast<const Binary*>(this)->two_level_hints());
}