@Deprecated
    @Override
    public final ImmutableSortedMap<K, V> buildKeepingLast() {
      // TODO(emcmanus): implement
      throw new UnsupportedOperationException(
          "ImmutableSortedMap.Builder does not yet implement buildKeepingLast()");
    }
  }

  private final transient RegularImmutableSortedSet<K> keySet;
  private final transient ImmutableList<V> valueList;