}

  public static <K extends Comparable<? super K>, V> ImmutableSortedMap<K, V> of(K k1, V v1) {
    return copyOf(singletonMap(k1, v1));
  }

  public static <K extends Comparable<? super K>, V> ImmutableSortedMap<K, V> of(
      K k1, V v1, K k2, V v2) {
    return new Builder<K, V>(Ordering.natural()).put(k1, v1).put(k2, v2).build();
  }