return (ImmutableBiMap<K, V>) RegularImmutableBiMap.EMPTY;
  }

  /** Returns an immutable bimap containing a single entry. */
  public static <K, V> ImmutableBiMap<K, V> of(K k1, V v1) {
    return new SingletonImmutableBiMap<>(k1, v1);
  }

  /**
   * Returns an immutable map containing the given entries, in order.
   *
   * @throws IllegalArgumentException if duplicate keys or values are added
   */