* Function<Integer, String> square = new Function<Integer, String>() {
   *     public String apply(Integer in) {
   *       return Integer.toString(in * in);
   *     }
   * };
   * Multimap<String, String> transformed =
   *     Multimaps.transformValues(multimap, square);
   *   System.out.println(transformed);
   * }</pre>
   *

   * backing set. The returned map does not support put operations.
   *
   * <p><b>Warning:</b> If the function rejects {@code null}, caution is required to make sure the
   * set does not contain {@code null}, because the view cannot stop {@code null} from being added
   * to the set.
   *
   * <p><b>Warning:</b> This method assumes that for any instance {@code k} of key type {@code K},