* (you can omit the comparator if it is the natural order).
   */
  public <F extends @Nullable Object> Ordering<F> onResultOf(Function<F, ? extends T> function) {
    return new ByFunctionOrdering<>(function, this);
  }

  <T2 extends T> Ordering<Entry<T2, ?>> onKeys() {
    return onResultOf(Entry::getKey);
  }

  /**