public static <K, V> ImmutableMap<K, V> of() {
    return (ImmutableMap<K, V>) RegularImmutableMap.EMPTY;
  }

  public static <K, V> ImmutableMap<K, V> of(K k1, V v1) {
    return ImmutableBiMap.of(k1, v1);
  }

  public static <K, V> ImmutableMap<K, V> of(K k1, V v1, K k2, V v2) {
    return new RegularImmutableMap<K, V>(entryOf(k1, v1), entryOf(k2, v2));
  }

  private final transient ImmutableBiMap<V, K> inverse;

  @SuppressWarnings("unchecked") // used only for the empty map, which works for any types
  RegularImmutableBiMap() {
    super(new RegularImmutableMap<K, V>(new HashMap<K, V>()));
    this.inverse = (ImmutableBiMap<V, K>) this;
  }

  RegularImmutableBiMap(ImmutableMap<K, V> delegate) {
    super(delegate);

   * code.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  @SuppressWarnings("unchecked")
  public static <K, V> ImmutableMap<K, V> of() {
    return (ImmutableMap<K, V>) RegularImmutableMap.EMPTY;
  }

  /**
   * Returns an immutable map containing a single entry. This map behaves and performs comparably to
   * {@link Collections#singletonMap} but will not accept a null key or value. It is preferable

    return RegularImmutableMap.fromEntries(entryOf(k1, v1), entryOf(k2, v2));
  }

  /**
   * Returns an immutable map containing the given entries, in order.
   *
   * @throws IllegalArgumentException if duplicate keys are provided
   */
  public static <K, V> ImmutableMap<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {