}

  /** Private constructor for inverse bimap. */
  private AbstractBiMap(Map<K, V> backward, AbstractBiMap<V, K> forward) {
    delegate = backward;
    inverse = forward;
  }

  @Override
  protected Map<K, V> delegate() {
    return delegate;
  }

  /** Returns its input, or throws an exception if this is not a valid key. */
  @CanIgnoreReturnValue
  @ParametricNullness
  K checkKey(@ParametricNullness K key) {

  }

  /** Private constructor for inverse bimap. */
  private AbstractBiMap(Map<K, V> backward, AbstractBiMap<V, K> forward) {
    delegate = backward;
    inverse = forward;
  }

  @Override
  protected Map<K, V> delegate() {
    return delegate;
  }

  /** Returns its input, or throws an exception if this is not a valid key. */
  @CanIgnoreReturnValue
  @ParametricNullness
  K checkKey(@ParametricNullness K key) {