* deserialization.
   */
  void setDelegates(Map<K, V> forward, Map<V, K> backward) {
    inverse = checkMapsAndMakeInverse(forward, backward);
    delegate = forward;
  }

  private AbstractBiMap<V, K> checkMapsAndMakeInverse(Map<K, V> forward, Map<V, K> backward) {
    checkArgument(forward.isEmpty());
    checkArgument(backward.isEmpty());
    checkArgument(forward != backward);