code: @CanIgnoreReturnValue
    public static <K, V> ImmutableMap<K, V> uniqueIndex(
        Iterable<V> values, Function<? super V, K> keyFunction) {
      // TODO(lowasser): consider presizing the builder if values is a Collection
      return uniqueIndex(values.iterator(), keyFunction);
    }
docs: Returns a map with the given {@code values}, indexed by keys derived from those values. In
  other words, each input value produces an entry in the map whose key is the result of applying
  {@code keyFunction} to that value. These entries appear in the same order as the input values.
  Example usage:
  
  <pre>{@code
  Color red = new Color("red", 255, 0, 0);
  ...
  ImmutableSet<Color> allColors = ImmutableSet.of(red, green, blue);
  
  Map<String, Color> colorForName =
  uniqueIndex(allColors, toStringFunction());
  assertThat(colorForName).containsEntry("red", red);
  }</pre>
  
  <p>If your index may associate multiple values with each key, use {@link
  Multimaps#index(Iterable, Function) Multimaps.index}.
  
  @param values the values to use when constructing the {@code Map}
  @param keyFunction the function used to produce the key for each value
  @return a map mapping the result of evaluating the function {@code keyFunction} on each value
  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