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

  public static <K, V> ImmutableBiMap<K, V> of(K k1, V v1) {
    checkEntryNotNull(k1, v1);
    return new SingletonImmutableBiMap<K, V>(k1, v1);
  }

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

/**
 * Implementation of {@code Entry} for {@link ImmutableMap} that adds extra methods to traverse hash
 * buckets for the key and the value. This allows reuse in {@link RegularImmutableMap} and {@link
 * RegularImmutableBiMap}, which don't have to recopy the entries created by their {@code Builder}
 * implementations.
 *
 * <p>This base implementation has no key or value pointers, so instances of ImmutableMapEntry (but