return keySet;
  }

  @Override
  public ImmutableSortedSet<K> descendingKeySet() {
    return keySet.descendingSet();
  }

  /**
   * Serialized type for all ImmutableSortedMap instances. It captures the logical contents and they
   * are reconstructed using public factory methods. This ensures that the implementation types
   * remain as implementation details.
   */