code: public static <K, V> ImmutableBiMap<K, V> of(K k1, V v1, K k2, V v2) {
      checkEntryNotNull(k1, v1);
      checkEntryNotNull(k2, v2);
      return new RegularImmutableBiMap<K, V>(new Object[] {k1, v1, k2, v2}, 2);
    }
docs: Returns an immutable map containing the given entries, in order.
  
  @throws IllegalArgumentException if duplicate keys or values are added