code: @NotNull
      public <U extends Comparable<? super U>> ComparatorCompat<T> thenComparing(
              @NotNull Function<? super T, ? extends U> keyExtractor) {
          return thenComparing(comparing(keyExtractor));
      }
docs: Adds the comparator, that uses a function for extract
  a {@link java.lang.Comparable} sort key, to the chain.
  
  @param <U> the type of the sort key
  @param keyExtractor  the function that extracts the sort key
  @return the new {@code ComparatorCompat} instance