code: public static <K, V> ImmutableMap<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
      checkEntryNotNull(k1, v1);
      checkEntryNotNull(k2, v2);
      checkEntryNotNull(k3, v3);
      return RegularImmutableMap.create(3, new Object[] {k1, v1, k2, v2, k3, v3});
    }
docs: Returns an immutable map containing the given entries, in order.
  
  @throws IllegalArgumentException if duplicate keys are provided