* Color red = new Color("red", 255, 0, 0);
   * ...
   * Iterator<Color> allColors = ImmutableSet.of(red, green, blue).iterator();
   *
   * 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