*     in the input collection to that value
   * @throws IllegalArgumentException if {@code keyFunction} produces the same key for more than one
   *     value in the input collection
   * @throws NullPointerException if any element of {@code values} is {@code null}, or if {@code
   *     keyFunction} produces {@code null} for any value
   */
  @CanIgnoreReturnValue
  public static <K, V> ImmutableMap<K, V> uniqueIndex(